I want to install Ubuntu, and I'm switching from Windows 10. I've heard that some people will check the integrity of the ISO image. Should I do this and if so, why?

I downloaded the ISO from , selecting the default "Ubuntu Desktop" option.  The website links the page  which gives instructions how to verify ubuntu.  This seems pretty tedious, and I am wondering how realistic it is that there is a problem with ISO downloaded from the official website. I note that the process of verification itself requires me to download software that is new to me, thus introducing another attack vector on me even as I am closing another one.  For what it's worth, I am planning to use Live USB only and not to fully install Ubuntu. Does that make a difference?


I recently was lucky enough to get a laptop for free. The only setback is it had no operating system. I burned ubuntu 12.04 LTS to a disk, and when i boot it in the laptop it loads. I tried installing it a few times, but during the installing process it says there was an input/output error. so after hitting retry a few times it still didnt work so i clicked ignore. While the actual installing process happens it says failed to create swap partition. I dont have any other os on it nor do i think i will (besides ubuntu). If anybody knows how to fix these errors i would be extremely happy.  Thank you in advance for your help

Today I downloaded Ubuntu for my laptop. It runs great from a bootable USB, but when I tried to install it, I got the following error message: The installer encountered an error copying files to the hard disk:  [Errno 5] Input/output error  This is often due to a faulty CD/DVD disk or drive, or a faulty hard    disk. It may help to clean the CD/DVD, to burn the CD/DVD at a lower   speed, to clean the CD/DVD drive lens (cleaning kits are often    available from electronics suppliers), to check whether the hard disk   is old and in need of replacement, or to move the system to a cooler   environment.     I tried everything to install it on my laptop including redownloading the Ubuntu ISO image, but nothing worked.


I cannot seem to save the value of a checkbox in a plugin I am creating. When I uncheck it and click Save Settings, it reverts to being checked again. I'm not sure where I have gone wrong. Any help would be most appreciated.   //Global Variable $options = array();  //Options Page     function iwmp_options_page() {          if( !current_user_can('manage_options') ) {             wp_die('You do not have sufficient permission to access this page.');         }          global $options;          if( isset($_POST['iwmp_settings_form_submitted']) ) {              $hidden_field = esc_html( $_POST['iwmp_settings_form_submitted'] );              if($hidden_field == "Y") {                  $iwmp_single_images = esc_html( $POST['iwmp_single_images'] );                  $options['iwmp_single_images'] = $iwmp_single_images;                  update_option('iwmp_settings', $options);              }          }          $options = get_option('iwmp_settings');          if( $options != '' ) {              $iwmp_single_images = $options['iwmp_single_images'];          }          require('plugin-settings-page-wrapper.php');      }   Inside the 'plugin-settings-page-wrapper.php' file: (Stripped down of course)  <label for="iwmp_single_images">Enable on Single Images:   <input checked="checked" name="iwmp_single_images" type="checkbox" id="iwmp_single_images" value="1" <? checked( $options['iwmp_single_images'], 1, false );?> /> </label>

I know this may be a duplicate however I haven't been able to make sense of previous questions.  I have a checkbox on a settings page. Everything is okay on the first load of the page, if I check the box and save all is fine. If I then uncheck i get the following error:     Notice: Undefined index: dat_checkbox_field_0 in .../wp-content/plugins/divi-auto-testimonials/admin/dat-options.php on line 49   value='1'>   The function:  function dat_checkbox_field_0_render(  ) {       $options = get_option( 'dat_settings' );     ?>     <input type='checkbox' name='dat_settings[dat_checkbox_field_0]' <?php checked( $options['dat_checkbox_field_0'], 1 ); ?> value='1'>     <?php  }   Line 49 is the input html.  I also get the same error for this code:  $options = get_option( 'dat_settings' ); if( $options['dat_checkbox_field_0'] != '1' ) {  include_once "admin/notification.php"; }   From what I understand I need to set the value as null I think but I am not entirely sure if that is correct and if so how.


I have a relatively old pc (5 years old) running on vista.  I got fed-up with vista and I am not quite convinced with windows 8. I seriously considering to switch to Ubuntu. Being quite new to the Linux/Ubuntu world I am not sure which is the best release for me. Shall I go to the latest release (the 13.10)? or shall I go for the latest LTS (the 12.04) release or something in between? Is it painful later on to upgrade your Ubuntu system? Please let me know your recommendations.  From System Requirements there should be no problem. My pc seems to have the suggested requirements for all the latest releases.  Thanks for your help.

For a given hardware configuration, how do I find out if Ubuntu will run on it? What considerations should I take into account when choosing an Ubuntu version and  such as:    with a lighter desktop than the usual Gnome and Unity  with the even lighter LXDE desktop   Obviously Ubuntu does not run on some processor architectures. So how do I go about choosing the right version and derivate. How can I find out the minmal system requirements?


I need a regexp that will match everything except a single constant (case ignored)  Example for constant ALL, should match words like: dog, MOUSE, mall, alligator. But it shouldn't match: all, ALL, alL.

I know it's possible to match a word and then reverse the matches using other tools (e.g. grep -v). However, is it possible to match lines that do not contain a specific word, e.g. hede, using a regular expression?   Input:  hoho hihi haha hede   Code:  grep "<Regex for 'doesn't contain hede'>" input   Desired output:  hoho hihi haha


I'm trying to load html from a MySQL database and display it via my php script. But it won't display foreign characters correctly.  I've already changed php.ini to have the default charset as utf-8 and I've set my php header to:  header('Content-Type: text/html; charset=utf-8');   And it still won't display the characters correctly. It's starting to really annoy me because I've tried every solution and none of them seem to be working.

I'm setting up a new server and want to support UTF-8 fully in my web application. I have tried this in the past on existing servers and always seem to end up having to fall back to ISO-8859-1.  Where exactly do I need to set the encoding/charsets? I'm aware that I need to configure Apache, MySQL, and PHP to do this — is there some standard checklist I can follow, or perhaps troubleshoot where the mismatches occur?  This is for a new Linux server, running MySQL 5, PHP, 5 and Apache 2.


I have the equation: $Q(x,y) = 5x^2 - 6xy + 5y^2$. The first question is to write this in  $ Q(z) = z^{T}Az$, in Matrixnotation.  The matrix I calculated is: $$\begin{bmatrix} 5 & -3 \\ -3 & 5 \end{bmatrix} $$  Then I have to diagonalize it, such that $A = UDU^{T}$ in order to use another Basis (from $z \to \overline{z}$).  [EDIT] I solved this part using the diagonalization, and obtaining $$\begin{bmatrix} 2 & 0 \\ 0 & 8 \end{bmatrix} $$ as the matrix for the new quadric. Now how can I solve the next question?  At the end I have to draw the new curve given by $z^{T}Az = 8$.  I tried to read some documentation but I don't understand how to procede, can someone help? Thanks!

I have this quadratic form  $Q= x^2 + 4y^2 + 9z^2 + 4xy + 6xz+ 12yz$  And they ask me:  for which values of $x,y$ and $z$ is $Q=0$?  and I have to diagonalize also the quadratic form.  I calculated the eigenvalues: $k_{1}=0=k_{2}, k_{3}=14$,   and the eigenvector $v_{1}=(-2,1,0), v_{2}=(1,2,3), v_{3}=(3,6,-5)$   I don't know if this is usefull in order to diagonalize or to see when is $Q=0$


In Django generic views, there's slug_field, and slug_url_kwarg. In this context, what is the definition of slug?  I choose the more persuasive explanation within items of 3 dictionaries.  In Cambridge dictionary:     A piece of metal used instead of a coin for putting in machines   In MW:      A disk for insertion in a slot machine; especially :one used illegally instead of a coin   In Oxford:     A part of a URL which identifies a particular page on a website in a form readable by users.   They don't seem to make sense.

When I read Django code I often see in models what is called a "slug". I am not quite sure what this is, but I do know it has something to do with URLs. How and when is this slug-thing supposed to be used?   (I have read its definition in .)


I got this classwork but got very confused with the form of answer the question needs, Can anyone please help? Thank you

I'm struggling a fair amount with this exercise:     Find a subring of $M(2,\mathbb{Q})$ which is isomorphic to a) $\mathbb{Q}$ x $ \mathbb{Q}$ b) $\mathbb{Q}$ c) $\mathbb{Q}[x]$/$x^2$   Now I know a subring must be a subgroup, must contain the elements $0,1$ and must be closed under multiplication. As we are looking for isomorphisms then they must also be ring homomorphisms and must be bijective.  I tried to come up with a random subring and attempt to prove it is an isomorphism. E.g $\left\{\left.\begin{matrix}  a & b \\ c & 0  \end{matrix}\right|a,b,c \in \mathbb{Q}\right\}$ is a subgroup of $M(2,\mathbb{Q})$ and then try and show it's an isomorphism. However I'm stuck here as I don't actually know how to go about finding these specific isomorphisms and I completely lose what I'm doing. Any help would be great.


I am not referring here to the process of Public Key exchange, but rather the process of encryption or decryption itself after the Public/Private keys have been generated. For instance, using Diffie-Hellman algorithm we ended up getting both Public and Private Keys of size 512 Bytes. How are we supposed then to use the Public Key to encrypt a given plain text message "Hello World!" and the Private Key to decrypt it?

One thing cryptography articles never seem to explain is how the message actually gets encrypted. You get this long-winded lecture on number theory which ends with, Ta Da! and we have a public and private key. Then, they never explain the process of exactly how some 100,000 byte word document gets encrypted using the public key.  Other times the explanations seem downright misleading. For example, in my book Cryptography (by Meyer and Matyas) it says for RSA the ciphertext is the plain text to the power of key. How do you exponentiate a word document? I don't get it.


i have this issue with javaFX GUI  ... i want when i press button i verify if there is no empty pane else i Add a new pane  .. but every time i get ConcurrentModificationException . i have tried with print and i see that only the first section of for here is executed and not the other   enter code here     button.setOnAction(new EventHandler<ActionEvent>() {                     @Override                     public void handle(ActionEvent event) {                         for (Node node : vbox.getChildren()) {                             if (node instanceof Pane){                                 Pane pane = (Pane) node ;                                 if (pane.getChildren().isEmpty()){                                     vbox.getChildren().remove(node);                                 }                             }``                         }                         drugrow drug = new drugrow();                         vbox.getChildren().addAll(drug.getPane());                     }                 });

We all know you can't do the following because of ConcurrentModificationException:  for (Object i : l) {     if (condition(i)) {         l.remove(i);     } }   But this apparently works sometimes, but not always. Here's some specific code:  public static void main(String[] args) {     Collection<Integer> l = new ArrayList<>();      for (int i = 0; i < 10; ++i) {         l.add(4);         l.add(5);         l.add(6);     }      for (int i : l) {         if (i == 5) {             l.remove(i);         }     }      System.out.println(l); }   This, of course, results in:  Exception in thread "main" java.util.ConcurrentModificationException   Even though multiple threads aren't doing it. Anyway.  What's the best solution to this problem? How can I remove an item from the collection in a loop without throwing this exception?  I'm also using an arbitrary Collection here, not necessarily an ArrayList, so you can't rely on get.


I have a struct:  struct fsobject { std::string fqname; //Fully Qualified Name of File incl extension std::string type;   //Type of Object, Either Directory or File std::string data;   //contents of file. Directories have no data. };   And a vector of type fsobject:  std::vector<fsobject> driveA;//Drive A   I am trying to iterate over the vector and access struct members for each vector element. This is the loop I have created:  for (auto vectorit = driveA.begin(); vectorit != driveA.end(); ++vectorit) {     cout << vectorit.fqname <<endl; }   Which results in the error:  main.cpp|50|error: ‘class __gnu_cxx::__normal_iterator >’ has no member named ‘fqname’|  I am having a hard time wrapping my head around how to accomplish accessing members of each vector element. Any help is appreciated.  Here is the full source:  #include <iostream> #include <algorithm> #include <string> #include <iostream> #include <list> #include <cctype> #include <vector>  struct fsobject { std::string fqname; //Fully Qualified Name of File incl extension std::string type;   //Type of Object, Either Directory or File std::string data;   //contents of file. Directories have no data. };   using namespace std;  bool compare(const fsobject& first, const fsobject& second) {   if (first.fqname < second.fqname)     return true;   else     return false; }  int main() {      std::vector<fsobject> driveA;//Drive A     std::vector<fsobject> driveB;//Drive B     std::vector<fsobject> driveC;//Drive C      driveA.push_back({"/home/zory/","dir",""});     driveA.push_back({"/home/dory/","dir",""});     driveA.push_back({"/home/","dir",""});     driveA.push_back({"/home/anakin/","dir",""});     driveA.push_back({"/home/luke/","dir",""});     driveA.push_back({"/home/luke/","dir",""});     driveA.push_back({"/home/luke/QuaterlyReports.pdf","file","bankruptcy is coming"});      sort(driveA.begin(),driveA.end(),compare);     /*cout << driveA.back().fqname << endl;     driveA.pop_back();     cout << driveA.back().fqname << endl;*/        for (auto vectorit = driveA.begin(); vectorit != driveA.end(); ++vectorit)     {         cout << vectorit.fqname <<endl;     }       cout << "Done...";      return 0; }   Thanks!

I think I've declared a Vector with an object correctly.  But, I don't know how to access it's members when looping with Iterator.  In my code, the line --->>  cout << " " << *Iter;  How do I print the contents of the members?  Like *Iter.m_PackLine ???  Not sure if I used the correct terminology, but appreciate the help!  Thanks  class CFileInfo {   public:       std::string m_PackLine;       std::string m_FileDateTime;       int m_NumDownloads; };  void main() {   CFileInfo packInfo;    vector<CFileInfo, CFileInfo&> unsortedFiles;   vector<CFileInfo, CFileInfo&>::iterator Iter;    packInfo.m_PackLine = "Sample Line 1";   packInfo.m_FileDateTime = "06/22/2008 04:34";   packInfo.m_NumDownloads = 0;   unsortedFiles.push_back(packInfo);    packInfo.m_PackLine = "Sample Line 2";   packInfo.m_FileDateTime = "12/05/2007 14:54";   packInfo.m_NumDownloads = 1;   unsortedFiles.push_back(packInfo);   for (Iter = unsortedFiles.begin(); Iter != unsortedFiles.end(); Iter++ )  {     cout << " " << *Iter; // !!! THIS IS WHERE I GET STUMPED     // How do I output values of the object members?   } }  // end main


I have this code: from bs4 import BeautifulSoup import requests import re import time import schedule   def scrape():      regex = r"\{.availability\":\"(?P<found>\w*\")"          URL = 'someurl'      headers = {         "User-Agent": 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:75.0) Gecko/20100101 Firefox/75.0'}      page = requests.get(URL, headers=headers)      soup = BeautifulSoup(page.content, 'html.parser')      scripts = str(soup.find_all('script'))      matches = re.finditer(regex, scripts, re.MULTILINE | re.DOTALL)      for matchNum, match in enumerate(matches, start=0):          print("{matchNum}: {match}".format(             matchNum=matchNum, start=match.start(), end=match.end(), match=match.group()))            schedule.every(3).seconds.do(scrape)   while 1:     schedule.run_pending()     time.sleep(1)  Code is checking whether an article is 'out_of_stock' or 'in_stock'. The problem is, I have to look like 24/7 at the consol waiting for the line 'in_stock' to appear. I was wondering, how to write a few lines of code to make a pop-up (?) notification saying: "Hey, 'in_stock" just popped-up!" Or I don't know, maybe a simple sound? (that's maybe even better) Any kind of notification. I would really appreciate any kind of help. I'm newbie to Python, so I'm sorry in advance if I don't know basic stuff. Have a wonderful evening! ~EDIT - I used this:

How can I play audio (it would be like a 1 second sound) from a Python script?  It would be best if it was platform independent, but firstly it needs to work on a Mac.  I know I could just execute the afplay file.mp3 command from within Python, but is it possible to do it in raw Python? I would also be better if it didn't rely on external libraries.


In many problem or theorem in which I have to show that given function is polynomial, there is the argument that for some k show that $f^{(n)}(z)=0, \forall n>k $ . I know that by definition of polynomial above characteristics is ok. But I am not convinced with fact that converse part. that there is no function other than polynomial which satisfies this property. Where am I  missing? Any help will be appreciated.

Let $f$ be a complex function with the property that $f^{(n)} = 0$.  Does this imply that $f$ is a polynomial?  If so, why?  Upon thinking about this problem myself, I can easily observe that every $n$ degree polynomial satisfies $f^{(n+1)} = 0$.  Still, this doesn't show that some non-polynomial function couldn't also have this property.


Let $X$ be a set such that $|X|=\aleph _0$. I need to find a family of sets $\mathcal{F}$, of subsets of $X$ such that $|\mathcal{F}|=|\mathbb{R}|$.  I saw a couple of examples of Specific X but I can't construct a family of sets which hold the condition for a general set X with $|X|=\aleph _0$.  edit: I forgot to mention an important condition that $\mathcal{F}$ must hold: for every $A,B\in \mathcal{F}$ the intersection $A\cap B$ is finite.

Can a countable set contain uncountably many infinite subsets such that the intersection of any two such distinct subsets is finite?


Need to somehow prove P(Z less then z) is less then equation2  I dont know where to get started with this problem. Please help me figure out how to go about doing this problem. Thank you  So do i need to just integrate the equation given in part a from z to infinity ... how does that show its greater then P(Z less then z)?

I know that an easy to handle formula for the CDF of a normal distribution is somewhat missing, due to the complicated error function in it.  However, I wonder if there is a a nice formula for $N(c_{-} \leq x < c_{+}| \mu, \sigma^2)$. Or what the "state of the art" approximation for this problem might be.


If I have a Word document and a CSV file was inserted inside the Word document, how can I tell the creation date of the CSV file and the person who created it when I open the CSV file up from within the Word document? (In other words, simple version control on the CSV file.)

Is there a possibility to search when the embedded txt file was created in the word document? I'm working in Word 2010 and the extension of the document is simple: .doc  It was inserted using the Word options:  Insert -> Object -> Create from file -> Insert as icon   Thank you, I already tried the right click and properties option, but the problem with it, that it shows only the current date for all of the 3 options (created/modified/accessed).           I also tried to get the timestamp but it is inactive when I'm on the txt object.   Sorry not to mention it before: Thank you all of you here for the help and support it is really appreciated.


Answers to coding questions often use a minor change to the example code. But because it's code, you cant emphasize those differences with markdown syntax.   See example below where I've done some gymnastics to show a diff:    Example Question  Why isn't his working?   <htwl> <body>     document </body> </html>     Example Answer  Ah, just a typo. You meant:     <htwl>   <html>       <body>           document       </body>   </html>     In the above example answer, I used a quote block instead of code block, and  <del> to strikeout the old line, and HTML character entities (including &nbsp; for indentation) to make sure the HTML didn't render as HTML. Painful.  I don't necessarily need to see a full diff, (as in git) where the old line is red and in ~strikeout~ and the new line is green. But it would be nice to give some sort of indication within a code block to emphasize or hi-light a particular change, to focus attention on that line. But of course, you cant use markdown inside of a code block!  That could really be anything: the ability to make a line bold, or a different color, or put a little arrow outside the left margin of that line. Anything to focus attention to certain lines.

Frequently when I ask a question I have to include quite a bit of code for reference purposes.    It would be nice to be able to bold the important lines in the code.  For example this code:  <DataTrigger Value="True">     <DataTrigger.Binding>         <MultiBinding Converter="{StaticResource DisableWorkItemConverter}">             <Binding ElementName="MainForm" Path="PickedWorkItemID"/>             <Binding Path="Id"/>         </MultiBinding>     </DataTrigger.Binding>     <Setter Property="IsEnabled" Value="False"/>     <Setter Property="IsSelected" Value="False"/> </DataTrigger>   Is not way long, but the important part (as seen in ) is the second to last line.  However, if I try to bold it, it ends up looking like this:  <DataTrigger Value="True">     <DataTrigger.Binding>         <MultiBinding Converter="{StaticResource DisableWorkItemConverter}">             <Binding ElementName="MainForm" Path="PickedWorkItemID"/>             <Binding Path="Id"/>         </MultiBinding>     </DataTrigger.Binding>     <Setter Property="IsEnabled" Value="False"/>     **<Setter Property="IsSelected" Value="False"/>** </DataTrigger>   Not what I was going for.  It would be nice to be able to draw attention to specific lines of code when needed.    (One suggestion of a different color background sounds nice too (ie a yellow background to draw the eye to the relevant code.)  If there is some other way to bold in code please let me know.


I am a second year Chemical Engineering student. However out of interest I wish to learn General theory of relativity. Can anyone please suggest some books for learning this subject?

What are some good books for learning general relativity?


I am using a 64GB flash drive with Ubuntu, and I still can read and write to it. But when I navigate to a certain folder is gives me the error message  Error when getting information for file '/media/alexander/D0f9-E846/Music/Misc/t....": Input/output error   Instead of the points there are some strange characters to see. I am not able to delete that directory.   The format is "W95 FAT32" and should stay in that format.   I assume formatting the flash drive will help only? Or is there another way of fixing this issue?

I'm using the FAT32 file system for my pen drive. It frequently has file/data corruptions.   In Windows, I used the scan disk utility to fix the FAT32/NTFS file systems. How can I do this in Ubuntu?


Down to what size is there experimental justification for modelling the electron as a point like particle without volume? Asked in another way, at what size scale would it be more correct to model the electron as a charged shell with unknown internal structure instead of a geometric point without volume?  [I am not questioning whether or not the model is correct - only where the cutoff between theory and proof should be drawn when evaluating alternative theories]  EDIT  The proposed duplicate question does not contain any answers that would suggest a 'proven maximum size' like I am asking about.

There is some confidence that electron is a perfect point e.g. to simplify QFT calculations. However, searching for experimental evidence (),  only points argument based on g-factor being close to 2:  extrapolating from proton and triton behavior that  for particles composed of 3 fermions should be $\approx g-2$:    Using more than two points for fitting this parabola it wouldn't look so great, e.g. neutron (udd) has $g\approx-3.8$ and .  And while classically $g$-factor is said to be 1 for rotating object, it is for assuming equal mass and charge density ($\rho_m\propto\rho_q$). Generally  by modifying charge-mass distribution:  $$g=\frac{2m}{q} \frac{\mu}{L}=\frac{2m}{q} \frac{\int AdI}{\omega I}=\frac{2m}{q} \frac{\int \pi r^2 \rho_q(r)\frac{\omega}{2\pi} dr}{\omega I}= \frac{m}{q}\frac{\int \rho_q(r) r^2 dr}{\int \rho_m(r) r^2 dr}$$  Another argument for point nature of electron is tiny cross-section, so  for electron-positron collisions:    Beside some bumps corresponding to resonances, we see a linear trend in this log-log plot: $\approx 10^{-6}$ mb for 10GeVs (5GeV per lepton), $\approx 10^{-4}$ mb for 1GeV. The 1GeV case means $\gamma\approx 1000$, which is also in Lorentz contraction: geometrically means $\gamma$ times reduction of size, hence $\gamma^2$ times reduction of cross-section - exactly as in this line on log-log scale plot.   More proper explanation is that it is for collision - transforming to frame of reference where one particle rests, we get $\gamma\to\approx \gamma^2$. This asymptotic $\sigma \propto 1/E^2$ behavior in colliders is well known () - wanting size of resting electron, we need to take it from GeVs to E=511keVs.  Extrapolating this line (no resonances) to resting electron ($\gamma=1$), we get $\approx 100$ mb, corresponding to $\approx 2$ fm radius.  From the other side we know that two EM photons having 2 x 511keV energy can create electron-positron pair, hence energy conservation doesn't allow electric field of electron to exceed 511keV energy, what  from $E\propto 1/r^2$:   $$\int_{1.4fm}^\infty \frac{1}{2} |E|^2 4\pi r^2 dr\approx 511keV$$  Could anybody elaborate on concluding upper bound for electron radius from g-factor itself, or point different experimental boundary?  Does it forbid electron's parton structure: being "composed of three smaller fermions" as Dehmelt writes? Does it also forbid some deformation/regularization of electric field to a finite energy?


I reset my windows 10 laptop . I am trying to log in now with my old username n password but it doesn’t work   I have try to reset it again but now it say “this laptop has no administration account , log in first n than try it again”  But how can I log in when my username n password is not working.  I am not able to log in by either local account nor Microsoft account.  I am certain that I remember my password and log in I’d.  I am not able to run any command like windows +R .  My laptop can’t detect USB too.   So how would I log my laptop now.   Please do suggest .

I got a brand new Windows 7 machine, installed the operating system, created one account and forgot its password. What can I do?  There is no external CD, the operating system is loaded from somewhere inside the machine.  I already tried to remember passwords and tried all candidates with all possible combinations of caps lock, num lock etc.


My ubuntu 12.10 is booting up in "low-graphic mode". All "executable" options lead to a flashing but unresponsive prompt (scroll/num/caps-lock is dead). Pressing the power button will cause a "normal" system-reboot in most of these options. "exit to console" will not.  How can I get my system back?  (reinstall is an option, but not ideal)  Possible troublemakers:   started setting up systemvpn, but did not complete config. ("NOVPN" in yellow text when rebooting) something went wrong when I installed/removed gnome3 my monitor is in fact EOL (takes some time for it to find its correct source, way past boot initz. x-configure is identifying it correctly though)

Note: This is an attempt to create a canonical question that covers all instances of "low-graphics mode" error that occurs to a user, including but not limited to installation of wrong drivers, incorrect or invalid lightdm greeters, low disk space, incorrect installation of graphics card like ATI and Nvidia, incorrect configuration of xorg.conf file while setting up multiple monitors among others. If you are experiencing the "low-graphics mode" error when trying to login but none of the following answers work for you, please do ask a new question and then update the answers of this canonical question as and when your new question gets answered.   When I try to boot into my computer, I am getting this error:  The system is running in low-graphics mode Your screen, graphics cards, and input device settings could not be detected correctly. You will need to configure these yourself.   How do I fix the failsafe X mode and login into my computer?  Answer index:


I want to prove that:  $$\text{ if } m,n \geq  1 \text{ and } m \mid n,\text{ then } \phi(m) \mid \phi(n).$$  How can I show this?  I thought the following:  $$m \mid n \Rightarrow \exists k \in \mathbb{Z} \text{ such that } n=km$$  $$\text{We write  } m \text{ like that : }m=p_1^{a_1} \cdots p_k^{a_k}, \text{ where } p_i \text{ are primes and } a_i>0$$  Then,we have $n=k p_1^{a_1} \cdots p_k^{a_k}$  We could show that $\phi(m) \mid \phi(n)$,using the fact that as $\phi$ is multiplicative and if  $(m,n)=1$ ,then $\phi(mn)=\phi(m) \phi(n)$  But..it is possible that $k \mid p_1^{a_1 \cdots p_k^{a_k}}$.  So,do I have to show maybe that $\gcd(kp_1^{a_1} \cdots p_{k-1}^{a_{k-1}},p_k^{a_k})=1$ ?

I want to prove that if $d \mid n \in \mathbb{N}$, then $\varphi(d) \mid \varphi(n)$.  It's given that $d \mid n$, so we know that $n = dm$, for some $m \in \mathbb{Z}$. Now, I want to show that $\varphi(d) \mid \varphi(n)$, i.e. that $\varphi(n) = k\varphi(d)$, for some $k \in \mathbb{Z}$. Drawing a blank of where to go from here.


I have several IP addresses that I want to list it based on internal or external IP: our internal IP for example start with 10.10.10.X, I was thinking about writing bash script to do this job for me, I tried to use the script below to read all IPs from text file.  while IFS='' read -r line || [[ -n "$line" ]]; do IP=$(echo "$line" | cut -c1-8) if [[ "$IP" == '10.10.10' ]]; then echo " $line" >> /tmp/internal.txt else echo " $line" >> /tmp/external.txt fi done < "$1"   but I faced the following problem (when debugging the code) when variable IP compared with 10.10.10 it will be shown as   [[ X.X.X. == \1\0\.\1\0\.\1\0 ]]   and the result always will be external, please advise what I did wrong.  Edit with example:  Let us say /tmp/ip.txt I have the following IPs.  10.10.10.1 192.168.5.1 20.30.5.1   After checking the IPs both 192.168.5.1 and 20.30.5.1 should go to /tmp/deny.txt and 10.10.10.1 should go to /tmp/allow.txt

Is there a script/program/utility already available for the following requirement in a optimised way?  someCommand |    tee >(grep "pattern" > LinesWhichMatch) |    grep -v "pattern" > LinesWhichDoesNotMatch


What can we clean camera sensors with?  Generally googling returns 'ethanol'.  Various people use ethanol for the job.  What else can we use?  Isopropyl alcohol, perhaps, eye glasses cleaner?  The sensors are actually covered by a protective layer and this is what we would be cleaning really.  What is this layer made of?  What is the main danger – what would be the worst, most dangerous chemicals to use for this job?

I've got a couple of spots of dust on the sensor of my Canon 20D. A rocket blower isn't shifting them, and I'm ridiculously scared of some of the more direct methods for cleaning sensors — what methods/systems have you tried, and how successful have they been?


My solution:   1) $ cl(A) \subseteq  cl(cl(A))$  let $x \in cl(cl(A))$ by definition $U_x \cap cl(A) \neq \varnothing $ $ \forall U_x \in Tau$  such that $x\in U_x $ .Thus $x \in cl(cl(A))$ $cl(A) \subseteq cl(cl(A))$ Is it true? And how prove other side.

I have to prove:   $\mbox{ClCl}A = \mbox{Cl}A$ $\mbox{diamCl}A = \mbox{diam}A$ If $G$ is open set then for any $A$ we have $ \mbox{Cl}(G \cap A) = \mbox{Cl}(G \cap \mbox{Cl}A)$   My attempt:   $\mbox{Cl}A \subseteq \mbox{ClCl}A$ is quite easy by fact that $A \subseteq \mbox{Cl}A $. But I don't have idea how can I prove that $ \mbox{ClCl}A  \subseteq \mbox{Cl}A$   But the point 2. and 3. is for me too difficult. Could you give me some hints? Thanks in advance!


I am new to all of this Linux based OS. Can I upgrade my Ubuntu 12.04.2 LTS OS to Ubuntu 12.10 and if so how do I go about doing that? I tried going through the update manager but the upgrade tab is not available after all updates are complete.

I am trying to upgrade to a new Ubuntu version but the update manager does not recognize an update. Why not?


What I would like to do is to put an LM386 in my bass guitar so that I can plug my headphones directly into it for practice. The LM386 was my first choice because it can run off of a 9V battery and requires no external parts. I understand that the output impedance of a 9V battery is fairly low (about an ohm or so). The LM386 has  PSRR of 50dB. Do I need to decouple the power supply going to the LM386?

Currently I run all my gadgets from batteries and don't use decoupling capacitors.  Are they generally needed/useful when drawing energy from a battery?


Is it possible to have a remote machine run a command, but redirect all reads and writes to a local machine.  Example: I have a bunch of scripts in folder 'project'. When 'make simulate' is run in 'project' a heavy simulation is run according to some input files and when done, output files are saved in the same folder. All this happens on my laptop, which is limited by battery and isn't as high-end as my desktop. Is there a way to have the simulation run on the desktop (for example via ssh) without going through the hassle of first copying the folder to the desktop, run the simulation there and then copy it all back? All relevant programs must of course be installed on the desktop.

Is it possible to do this:  ssh user@socket command /path/to/file/on/local/machine   That is to say, I want to execute a remote command using a local file in one step, without first using scp to copy the file.


In reference to this question:   Where in a published D&DAL Module (I'm specifically working with the Tyranny of Dragons modules) would I find the part where it says the expected duration/runtime? All it says is the XP and Downtime rewards for the adventure, but the DM's guide for AL says that their new rules trump the old guidelines, and that I gain XP & GP based on the amount of time the adventure is meant to run, as well as the level of the party.  Alternatively, if it is not written in said modules, how would one go about calculating or otherwise determining what time duration to use when calculating these DM rewards?

I recently started DMing Adventurers League sessions, and I've been told about DM rewards, being particularly interested in the XP I get after each DMed session.  Reading the AL DM guide, I understand that the XP is given in accordance to [a value corresponding to the average party level] × [a number of hours]. It is said that, for Hardcover adventures, the number of hours is the real amount of time the game took, but for modules, it's supposed to be the prescribed amount of time.  For the first adventure I DMed, there was indeed a prescribed duration (2 hours), but then, the next two ones didn't have any (I read the entire adventures, and nowhere would it say "Part X should take you Y hours to do" or anything similar). They were respectively from season 1 and 2 (DDEX01-04 “Dues for the Dead” and DDEX02-02 “Embers of Elmwood”), probably explaining the lack of duration, as I suppose they hadn't started to put in durations at that time.  So, do I just consider modules without prescribed play time as Hardcover adventures for the purpose of calculating my DM XP reward, or is there something I'm missing?


I am using Ubuntu 12.04.5 LTS Release:        12.04 Codename:       precise    I want to install Virtualmin on my VPS, but I am getting errors while installating. Check the below errors:  INFO - Removing Debian standard Webmin package, if they exist... INFO - Removing Debian apache packages... INFO - Installing dependencies using command: /usr/bin/apt-get --config-file apt.conf.noninteractive -y --force-yes install bsdutils postfix postfix-pcre webmin usermin ruby libxml-simple-perl libcrypt-ssleay-perl unzip zip libfcgi-dev bind9 spamassassin spamc procmail procmail-wrapper libnet-ssleay-perl libpg-perl libdbd-pg-perl libdbd-mysql-perl quota iptables openssl python mailman subversion ruby irb rdoc ri mysql-server mysql-client mysql-common postgresql postgresql-client awstats webalizer dovecot-common dovecot-imapd dovecot-pop3d proftpd libcrypt-ssleay-perl awstats clamav-base clamav-daemon clamav clamav-freshclam clamav-docs clamav-testfiles libapache2-mod-fcgid apache2-suexec-custom scponly apache2 apache2-doc libapache2-svn libsasl2-2 libsasl2-modules sasl2-bin php-pear php5 php5-cgi libapache2-mod-php5 php5-mysql ...in progress, please wait... .E: Unable to correct problems, you have held broken packages.  /usr/bin/apt-get --config-file apt.conf.noninteractive -y --force-yes install bsdutils postfix postfix-pcre webmin usermin ruby libxml-simple-perl libcrypt-ssleay-perl unzip zip libfcgi-dev bind9 spamassassin spamc procmail procmail-wrapper libnet-ssleay-perl libpg-perl libdbd-pg-perl libdbd-mysql-perl quota iptables openssl python mailman subversion ruby irb rdoc ri mysql-server mysql-client mysql-common postgresql postgresql-client awstats webalizer dovecot-common dovecot-imapd dovecot-pop3d proftpd libcrypt-ssleay-perl awstats clamav-base clamav-daemon clamav clamav-freshclam clamav-docs clamav-testfiles libapache2-mod-fcgid apache2-suexec-custom scponly apache2 apache2-doc libapache2-svn libsasl2-2 libsasl2-modules sasl2-bin php-pear php5 php5-cgi libapache2-mod-php5 php5-mysql failed.  Error (if any): 0  Displaying the last 15 lines of /root/virtualmin-install.log to help troubleshoot this problem: sasl2-bin is already the newest version. unzip is already the newest version. php5 is already the newest version. php5-cgi is already the newest version. php5-cgi set to manually installed. php-pear is already the newest version. Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation:  The following packages have unmet dependencies:  libapache2-mod-fcgid : Depends: apache2.2-common but it is not going to be installed  libapache2-svn : Depends: apache2.2-common but it is not going to be installed  FATAL - Fatal Error Occurred: Something went wrong during installation: 0 FATAL - Cannot continue installation. FATAL - Attempting to remove virtualmin repository configuration, so the installation can be FATAL - re-attempted after any problems have been resolved. FATAL - Removing temporary directory and files. FATAL - If you are unsure of what went wrong, you may wish to review the log FATAL - in /root/virtualmin-install.log   I have also tried apt-get update and apt-get -f install. I have removed Apache but no success. Please help me out with this issue.

After upgrading from 10.04 to 12.04 I am trying to install different packages.  For instance ia32-libs and skype (4.0).  When trying to install these, I am getting the 'Unable to correct problems, you have held broken packages' error message.   Output of commands:  sudo apt-get install -f Reading package lists... Done Building dependency tree        Reading state information... Done 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.   After running this:  sudo dpkg --configure -a foo@foo:~$ sudo apt-get install -f Reading package lists... Done Building dependency tree        Reading state information... Done 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.


What are the best practices when designing a form in which some inputs can be used to calculate other inputs?  I am designing a form with about a dozen text inputs. The first three of these inputs are required. However, if a user fills out two of the optional inputs we can calculate one of the required inputs.

This is a problem I've ran into several times, and I imagine that others have ran across it too.    Basically, I have a complicated equation, and I want the user to type in the values needed to calculate the results.  (For what it's worth, there are good default values for the variables in the equation.)  I've constructed a GUI that shows the user the names of the variables/fields that need to be provided, along with a control (like a numeric up/down, slider, etc.) to enter the values of the field.  Below that, I have an area that shows the output of the equation.  So my question is this.  Should my results area automatically change the calculated results whenever the user makes any changes to any of the input fields, or should they push a "Calculate" button to generate the results?  One other thing that probably applies is that the user is not trying to "get the right answer" out of the results.  They shouldn't be using the output to then go back and modify the input to get a better answer.  Are there any benefits to requiring the button press?  Is the extra click to see the results worth those benefits?


I am looking to understand the intuition behind moments of random variables.  I understand the first moment relating to the mean and the second one relating to the variance. But what use does the n-th moment have?   Thank you

While I understand the intuition behind expectation, I don't really understand the meaning of variance and moment.   What is a good way to think of those two terms?


To which of the four forces of nature does the coupling of the Higgs scalar field to other quantum fields ultimately belong to, if any? If it doesn't fall into any of the four known categories of forces, does that imply a fifth "force" of nature, or have I misunderstood the definition of force?

When I first learned about the four fundamental forces of nature, I assumed that they were just the only four kind of interactions there were. But after learning a little field theory, there are many other kinds of couplings, even in the standard model. So why isn't the Yukawa Higgs coupling considered a fifth fundamental force? To be a fundamental force, does there need to be a gauge boson mediating it?


I Know it is near duplicate. But I am looking for a little more solid answers.  I wondered always, is there any way to retain the order in LINQ To Objects. While searching for the solution, fortunately I found,  . I have gone through it.  Also, I searched on MSDN sites, but there is no such remarks/words which confirms the word "Order Preservation".  My question is, however they will also loop through each and every element to process it. Sequential traversal will not be a overhead when compared to accessing the elements randomly.    If not, Whether they have applied any sort of random algorithm to access the elements? Since, no word is specified about order in MSDN, which means, in future they can change implementation, so I should not ever trust LINQ methods regarding the order.   Is there any way to confirm the order of the retrieval.  Thanks.

I use LINQ to Objects instructions on an ordered array.  Which operations shouldn't I do to be sure the order of the array is not changed?


I have an app running that is showing in my statusbar icon menu, but it's rainbowing - I can't click on it or interact with it in any way.  The same application doesn't show in my Force Quit Applications menu.  What choices do I have?

What are all the ways to kill a buggy GUI app?     I can't right-click on its icon in the whatever-its-called bar, or use the top-of-window menu.  The app does not respond to anything, and does nothing but eat more and more memory as times goes on.  In Linux, there's xkill.  Is there something similar on OSX?


when i am installing the softwares by all means (via software center, terminal etc) it gives an error message as "Package dependencies cannot be resolved" Can any one help me out to solve the problem??  Error in Detail  "This error could be caused by required additional software packages which are missing or not installable. Furthermore there could be a conflict between software packages which are not allowed to be installed at the same time."

Occasionally, when I'm installing stuff, I get an error like the following:  Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation:  The following packages have unmet dependencies:  package1 : Depends: package2 (>= 1.8) but 1.7.5-1ubuntu1 is to be installed E: Unable to correct problems, you have held broken packages.   How can I resolve this?


I've recently gotten into physics, since I wanted to start learning it after I self-studied Calculus 1. I'm currently using the textbook "Fundamentals of Physics" by Halliday, however, I'm having a bit of a problem with vectors.   I think I have a bit of an intuitive understanding with the dot product, but not a strong one. On the note of cross product, I'm pretty clueless on how it works intuitively with the use of the right-hand rule.   I was wondering if someone could point me to any resources or give me an explanation that could try and give me a stronger intuitive sense for both? Thank you in advance.

I get the physical significance of vector addition & subtraction. But I don't understand what do dot & cross products mean?   More specifically,    Why is it that dot product of vectors $\vec{A}$ and $\vec{B}$ is defined as $AB\cos\theta$? Why is it that cross product of vectors $\vec{A}$ and $\vec{B}$ is defined as $AB\sin\theta$, times a unit vector determined from the right-hand rule?   To me, both these formulae seem to be arbitrarily defined (although, I know that it definitely wouldn't be the case).   If the cross product could be defined arbitrarily, why can't we define division of vectors? What's wrong with that? Why can't vectors be divided?


I have iPad 3 iOS 5.1.1 Jailbroken.  I now want to update to 7.0.4 or 7.0.6 but can NOT figure out how. (7.1.1 is currently the official update)  Manual updates thru iTunes to a 7.0.6 restore file always result in "This device isn't eligible" error despite hosts editing. I've tried on multiple computers.  Am I hosed?

I'm trying to downgrade my iPhone from the current iOS to an older one, as per .  But every time I try, I always receive this error:     The iPhone could not be restored. This device isn't eligible for the requested build.     I have tried many times with and without DFU mode. Any ideas regarding this?


What is the best way to add a custom WYSIWYG to custom objects?  So instead of this   We want to see this:

I've noticed that the WYSIWYG editor for sending HTML emails in a SalesForce case is dated (doesn't support keyboard shortcuts, doesn't support text indentations, doesn't support font styles/headers etc).    The editor toolbar for knowledge base article appears newer and has more features.    Is there an easy way to tell SalesForce to use a newer text formatting toolbar for sending emails?


After reading  and , I want to remove the title attribute from all links of my blog, as that is good from accessibility point of view. How this can be achieved?  UPDATE  I found a  how to remove title attribute from images, but it doesn't work for me.

I'm looking for a way to remove the title attribute from images, preferably through using a hook or a filter.  Most clients don't bother giving images a proper title, thus eliminating the purpose of this attribute. I prefer no tooltips to random, non-descriptive ones.


While Checking the /var/log/secure on my centos server have found lot of attempts of failed login by using unknown usernames, from a list of ip from Japan And China  How to hide my server from these smart guys or tools ;-)  Here is the snippet of log       Aug 27 12:07:06 EEHB-VM1 sshd[1191]: subsystem request for sftp Aug 27 12:08:09 EEHB-VM1 sshd[1191]: pam_unix(sshd:session): session closed for user root Aug 27 12:24:03 EEHB-VM1 sshd[1375]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=remember-template.com.tw  user=root Aug 27 12:24:04 EEHB-VM1 sshd[1377]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=remember-template.com.tw  user=root Aug 27 12:24:06 EEHB-VM1 sshd[1377]: Failed password for root from 106.186.113.82 port 7176 ssh2 Aug 27 12:24:07 EEHB-VM1 sshd[1378]: Connection closed by 106.186.113.82 Aug 27 12:24:07 EEHB-VM1 sshd[1375]: Failed password for root from 106.186.113.82 port 7176 ssh2 Aug 27 12:24:07 EEHB-VM1 sshd[1376]: Connection closed by 106.186.113.82 Aug 27 12:49:31 EEHB-VM1 sshd[1883]: Invalid user fluffy from 106.186.113.82 Aug 27 12:49:31 EEHB-VM1 sshd[1884]: input_userauth_request: invalid user fluffy Aug 27 12:49:31 EEHB-VM1 sshd[1883]: pam_unix(sshd:auth): check pass; user unknown Aug 27 12:49:31 EEHB-VM1 sshd[1883]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=remember-template.com.tw  Aug 27 12:49:31 EEHB-VM1 sshd[1883]: pam_succeed_if(sshd:auth): error retrieving information about user fluffy Aug 27 12:49:31 EEHB-VM1 sshd[1885]: Invalid user fluffy from 106.186.113.82 Aug 27 12:49:31 EEHB-VM1 sshd[1886]: input_userauth_request: invalid user fluffy Aug 27 12:49:31 EEHB-VM1 sshd[1885]: pam_unix(sshd:auth): check pass; user unknown Aug 27 12:49:31 EEHB-VM1 sshd[1885]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=remember-template.com.tw  Aug 27 12:49:31 EEHB-VM1 sshd[1885]: pam_succeed_if(sshd:auth): error retrieving information about user fluffy Aug 27 12:49:33 EEHB-VM1 sshd[1883]: Failed password for invalid user fluffy from 106.186.113.82 port 53242 ssh2 Aug 27 12:49:33 EEHB-VM1 sshd[1884]: Connection closed by 106.186.113.82 Aug 27 12:49:34 EEHB-VM1 sshd[1885]: Failed password for invalid user fluffy from 106.186.113.82 port 45149 ssh2 Aug 27 12:49:34 EEHB-VM1 sshd[1886]: Connection closed by 106.186.113.82   This behaviour is very frequent and being repeated within short span of time.  I suspect this as some fraudulent access attempt, and would like to know how should i deal with them.  Interestingly : how did they came to know the ip address even when the website isn't even live and have disallowed robots too.   EDIT 1: May be i can select these ip address and restrict them from firewall. But Is there a way i dont even let them try this mischievous attempt rather going to access logs daily and adding couple of other rules in firewall ?  P.S. : Appreciate if someone can give me a sample iptable rule snippet for now to deal with bunch of these IPs ;-)

What tool or technique do you use to prevent brute force attacks against your ssh port. I noticed in my Security logs, that I have millions of attempts to login as various users through ssh.  This is on a FreeBSD box, but I imagine it would be applicable anywhere.


I downloaded Fallout 3 iso file and mounted and there isn't any files or folders. Size of iso file is 5.9 GB

I'm running into trouble trying to mount a large iso:  dev@dev-OptiPlex-745:~$ sudo mount -o loop /home/dev/Hämtningar/matlab2011a_64.iso /cdrom mount: warning: /cdrom seems to be mounted read-only. dev@dev-OptiPlex-745:~$    Can you tell me how I should do it?


I run into these moments every now and then where I want to make a major update to my website but then I feel I just cant do it because search engines crawl it at that time.   Is there a way I can make the search engines temporarily stop crawling my site instantly while I make updates to it? (Yes, I'm looking for that "Off" and "On" switch)  I feel if I make updates while search engines crawl it, then I'll probably lose rankings in search.  ** Update **  I'm more interested in the fraction of time that the upload takes place. I'd like to ideally arrange a time with the bots to stop crawling at that point (between the time upload starts and the time upload ends). I'm not asking for a solution for a long-term downtime.

Currently, my website is under maintenance. I want to be the only person who's able to access/view the website online until I finish maintenance. How to set restrictions to others? I hate stalkers.


In my quantum mechanics textbook it says that the relation between the basis $|x\rangle$ and $|p\rangle$ is given by:  $\langle p | x \rangle = \Large \frac{e^{-ip x/ \hbar}}{\sqrt{2\pi \hbar}} \, .$  However, i'm not sure how to go about proving this relation. My idea was that the eigenstates of momentum can be written as below:  $\phi(x,p) = \Large \frac{1}{\sqrt{2\pi \hbar}} e^{-ipx/\hbar},$  which form an orthogonal basis.  But, we can expand any state, $|\text{state}\rangle$, in terms of the othorgonal basis right? So is my notation below correct for the expansion of the state $| x \rangle$ ?  $|x\rangle = \int_{-\infty}^{+\infty} \phi(p', x) |p'\rangle dp'$  If it were correct then the following would also be correct?  Using the fact that $\langle p | p' \rangle = \delta(p - p')$, $\langle p | x \rangle$ can be computed:  \begin{align} \langle p | x \rangle =& \int_{-\infty}^{+\infty} \phi(p', x) \langle p | p' \rangle dp' \\ =& \int_{-\infty}^{+\infty} \phi(p', x) \delta(p - p') dp' \\ =& \phi(p, x) \\ =& \Large \frac{e^{-ip x/ \hbar}}{\sqrt{2\pi \hbar}} \end{align} as required.  Does the logic work here?

I'm curious whether I can find the overlap $\langle q | p \rangle$ knowing only the following:   $|q\rangle$ is an eigenvector of an operator $Q$ with eigenvalue $q$. $|p\rangle$ is an eigenvector of an operator $P$ with eigenvalue $p$. $Q$ and $P$ are Hermitian. $[Q,P] = i \hbar$.   I'm asking because books and references I've looked in tend to assume that $Q$ is a differential operator when viewed in the $P$-basis, then show that it satisfies the commutation relation. (I haven't read one yet that proves that the form given for $Q$ is the only possible one.) I've heard, though, that we can work purely from the hypothesis of the commutation relation and prove the properties of $Q$ and $P$ from it.


I'm looking for the title of a book I read as a kid in the 60s/70s. It's about a teenage boy who's fascinated with magic and Harry Houdini, and develops his magical powers with Houdini's help.  I just found the book ---Black and Blue Magic. It's about a 12 year old kid named after Harry Houdini, and he grows wings through use of a magic potion. I highly recommend this for young adults.

I'm trying to remember the name of a novel I read as a pre-teen.  If I recall correctly, it would have come out in the 90s (though it may have been a reprint).  The novel is set in modern times.  I recalled this because I've been reading a lot of urban fantasy lately, and this may be the first 'urban fantasy' book I read - our world, but with a touch of magic.  There's a somewhat clumsy, out of shape teenaged (or almost teenaged) boy who doesn't have a father.  His father had been a stage magician who was constantly frustrated by his son's complete inability to preform stage magic, despite a prediction saying he would have a great or powerful magic.  His father had died before the book started, I don't recall how long before.  The boy does something nice for an old woman (I don't recall what) and she gifts him a magical potion.  A drop of it, rubbed on his shoulder blades, will make him grow wings.  The book follows him for the summer, as he spends his evenings flying.  He makes himself a toga-like shirt out of some old curtains and spends time flying around at night.  I recall one scene where he learns about two children out in the middle of the bay (perhaps the book is set in San Fran?  I don't recall) in a rowboat with one oar.  He saves them (by pulling them back to shore using a rope they had with them) and gets mentioned in the paper as an angel.  At the end of the book, the potion has run out.  There's only a tiny little bit left, and when he rubs it in, it creates only a single feather.  Around that point, the boy realizes that he's no longer getting winded going up several flights of stairs and he's gained a lot of self-confidence.  From what I remember of the copy I read, the cover featured a shirtless boy wearing jeans standing in front of a mirror, looking at the large white feathered wings that had grown from his back.


On line 7&8 I am trying to set a group of object values according to an input array, but the property for all of the objects is set to 10 - the value in the last iteration of the loop. If somebody could explain why this isn't working how I would expect it to that would be amazing.  I am currently learning python, so I have no idea if there is an easier way to do this.  class Network:     def __init__(self, input_data, layers, nodes, output_length):         self.layer = [Layer(len(input_data))]         self.layer += [Layer(nodes)] * layers         self.layer += [Layer(output_length)]         self.input = input_data         for __i in range(len(self.input)):             self.layer[0].nodes[__i].value = self.input[__i]          for self.i in range(1, self.layer.__len__()):             for self.j in range(0, self.layer[self.i].nodes.__len__()):                 self.layer[self.i].nodes[self.j].connection = [None] * self.layer[self.i - 1].nodes.__len__()      def compute(self):         for self.i in range(1, len(self.layer)):             for self.j in range(0, len(self.layer[self.i].nodes)):                 for self.k in range(0, len(self.layer[self.i].nodes[self.j].connection)):                     self.layer[self.i].nodes[self.k].value += self.layer[self.i-1].nodes[self.k] * self.layer[self.i].nodes[self.j].value[self.k]   class Layer:     def __init__(self, nodes):         self.nodes = [NodeObject()] * nodes   class NodeObject:     def __init__(self):         self.value = 0         self.connection = [None]      def set_value(self, value):         self.value = value   i = Network([1,2,3,4,5,6,7,8,9,10], 2, 16, 10) print(i.layer[0].nodes[2].value)   Thanks in advance, Matt

I needed to create a list of lists in Python, so I typed the following: my_list = [[1] * 4] * 3  The list looked like this: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]    Then I changed one of the innermost values: my_list[0][0] = 5  Now my list looks like this: [[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]    which is not what I wanted or expected. Can someone please explain what's going on, and how to get around it?


I'm looking for a book or short story where the protagonists are two life forms of a species that spends its time playing a game. The game involves them setting evolution parameters to create simulated civilizations, and compete into which one evolves the most. These "games" are broadcast, the signals picked up by a much more advanced civilization which mistakes these games as real involvement, and (maybe) eliminates the protagonist's civilization as a response?  I don't remember exactly when I've read the book, but it isn't the Culture book (The Player of Games). What made it memorable was the premise of running simulations in a VR environment, which are broadcast, and can be seen by another civilization as a report on the destruction of another world. Many levels of existence in the same book. Or it could be a short story, not sure...

Things I remember   I read this book in elementary school, but I don't know how much of a "kid's" book it is. Main character was male, and I think the title of the book may have been his name. I picture him as owl-like, but I know he had talons and wings at least. The book possibly started on his race's home planet, and they had to leave to live on space stations for some reason. Once they leave their home planet, there is a comment that their children will be born with flatter feet to accommodate walking on metal of the space station instead of gripping branches. I'm forgetting a chunk of the book, but the main character becomes separated from everyone else, possibly left behind somewhere on purpose. Somehow the main character becomes extremely powerful. He is able to fly through space on his own without a ship. Near the end of the book, he engages in a prolonged battle with some other hyper-powerful character (The fight might start on a water planet). Their fight spans the galaxy. Main character ends up in a black hole, omniscient and observing a planet (possibly Earth?). A boy from the planet dies and shows up where our main character is. Main character and boy talk about whether anything in life has meaning or affects anything.


I have two classes. Rectangle.java and TestRectangle.java. I have to use those two classes to draw a simple 2D rectangle with javafx.fxml.FXML, GraphicsContent2D. Here's what I have so far. Rectangle.java  import javafx.fxml.FXML; import javafx.scene.canvas.Canvas; import javafx.scene.canvas.GraphicsContext;   public class Rectangle {     @FXML private Canvas canvas;       public void draw() {         // get the GraphicsContext, which is used to draw on the Canvas         GraphicsContext gc = canvas.getGraphicsContext2D();          // clear canvas for next drawing          gc.clearRect(0, 0, canvas.getWidth(), canvas.getHeight());          // draw rectangle          gc.strokeRect(100, 100, 175, 300);     }  }  and the second, TestRectangle.java: TestRectangle.java  public class TestRectangle {     public static void main(String[] args) {         Rectangle rectangle1 = new Rectangle();         rectangle1.draw();      } }  When I run the program, I get the error Exception in thread "main" java.lang.NullPointerException     at Rectangle.draw(Rectangle.java:13)     at TestRectangle.main(TestRectangle.java:4)  I'm brand new to Java, and I'm sure this is a very simple issue. Any help would be appreciated! I also want to do the same for a Sphere and Circle. I'm guessing that I'm missing a declaration somewhere, but can't find what it may be.

What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?


An iPhone XS has been subject water to both physical and heat damage, however, the photos are still accessible when connected to a computer. Can physical damage to an iPhone also affect the quality of the photos taken on it?

I dropped my iPhone Xs into a hot, steaming dishwasher, and it seems to be working fine. However, the photos on it are very grainy and pixelated. Could hot steam damage the data on an iPhone X?


If I have a class as follows:  class A {   private int _xyz;   public int XYZ   {      get { return _xyz; }      set { _xyz = value; }          } }   Is there any use in defining a property or does it make sense to just make '_xyz' public?

I see a lot of example code for C# classes that does this:  public class Point {     public int x { get; set; }     public int y { get; set; } }   Or, in older code, the same with an explicit private backing value and without the new auto-implemented properties:  public class Point {     private int _x;     private int _y;      public int x {         get { return _x; }         set { _x = value; }     }      public int y {         get { return _y; }         set { _y = value; }     } }   My question is why. Is there any functional difference between doing the above and just making these members public fields, like below?  public class Point {     public int x;     public int y; }   To be clear, I understand the value of getters and setters when you need to do some translation of the underlying data. But in cases where you're just passing the values through, it seems needlessly verbose.


I am getting this error code:  Exception in thread "Thread-2" java.lang.NullPointerException at com.game.src.main.SpriteSheet.grabImage(SpriteSheet.java:15) at com.game.src.main.Player.<init>(Player.java:23) at com.game.src.main.Game.init(Game.java:37) at com.game.src.main.Game.run(Game.java:83) at java.lang.Thread.run(Unknown Source)   Here is init() from game  public void init() {     handler = new Handler();     handler.addObject(new Player(200,(370-64),this,1,ObjectId.Player1));     BufferedImageLoader loader = new BufferedImageLoader();     try     {         spriteSheet = loader.loadImage("/spritesheet.png");         background = loader.loadImage("/background.gif");     }catch(IOException e)     {         e.printStackTrace();     }      addKeyListener(new KeyInput(handler, this));     this.requestFocus();   }   The constructor for Player:  public Player(float x, float y, Game game, int playerX, ObjectId id) {     super(x,y,id);     this.game = game;     SpriteSheet ss =  new SpriteSheet(game.getSpriteSheet());     if (playerX == 1)     {         player = ss.grabImage(1,1,32,32);     }     else player = ss.grabImage(4,1,32,32);     gravity = 0.2;     maxDY = 5; }   and Class SpreadSheet  package com.game.src.main; import java.awt.image.*; public class SpriteSheet { private BufferedImage image; /**  * Constructor for objects of class SpriteSheet  */ public SpriteSheet(BufferedImage image) {     this.image = image; }  public BufferedImage grabImage(int col, int row, int width, int height) {     BufferedImage img = image.getSubimage((col*32) - 32, (row*32) - 32, width, height);     return img; } }   And GameObject():  package com.game.src.main;  import java.awt.Graphics; import java.awt.Rectangle; import java.util.LinkedList;  public abstract class GameObject {   protected float x,y; protected ObjectId id; protected float velX = 0, velY = 0;  public GameObject(float x, float y, ObjectId id) {     this.x = x;     this.y = y;     this.id = id; }    public abstract void tick(LinkedList<GameObject> object); public abstract void render(Graphics g); public abstract Rectangle getBounds(); public float getX() {     return x; } public  float getY() {     return y; }  public void setX(float x) {     this.x =x; } public void setY(float y) {     this.y = y; } public void setVelX(float velX) {     this.velX = velX; } public void setVelY(float velY) {     this.velY = velY; }  public ObjectId getId() {     return id; }  public void jump(double jumpHeight, boolean canJump) {     if (canJump)         velY -= jumpHeight; }   }  My project path is:  >1v1    >src    >res   This was working before but when I made Player a GameObject it stopped working. Any ideas?


I recall reading an American SF short story which I read in my early teens. It made quite an impression on me. It starred a couple, a man of high intelligence, who every ten minutes was subjected to a burst of painful static, and his wife, a beautiful woman who was forced to wear ugly clothing.  At the time I took it to be a satire of communism, given that it was written by an American during the Cold War era. Although now, I wonder given a world of savage inequalities if it wasn't in fact being prophetic.  I don't recall either the author or the title of the story. Any help would be appreciated in identifying the author and the short story.

I read this book in Middle School and I can't for the life of me remember the title, but I can remember what it was about for the most part:  Takes place in the future, the government imposes laws that outlaw people from being different in appearance and skill level. For example, if you are fast or strong or tall, they will put weights on you to weigh you down so that you'll be more slower/shorter and more like the average. Another example, if you are pretty the government issues you "ugly glasses" etc.  Basically, it's all about making sure everyone is equal and punishing the extraordinary.  One of the main characters (a man) decides to literally loose himself of his shackles and weights, breaking them to display his power (if I'm remembering correctly).   Any ideas folks?


As far as I can tell, writing \ast and * yield the same latex output.  Are there any subtle differences between these two symbols?  Are there any reasons to prefer one over the other?  Do they produce literally the same output or are the symbols slightly different in some fonts?  Is one or the other compatible with more latex compilers?  Does their interchangeability depend on being in math mode, or not being in math mode?

When you read a formula aloud you say "r and r-star are related by this formula". If you write this formula down you normally write an r with a six-handed star, which would actually be an asterisk and not star (with 5 sharp hands) because it's easier to draw by hand.  But what to do when typing this formula in LaTeX? Should I set it as I pronounce it?  r^\star = a r   Or as I write it by hand?  r^\ast = a r   And will \ast always do the same as *?  r^\ast == r^* %???   Is one (\star or \ast) simply wrong (typographically or semantically) or is it just a matter of personal preference?


Hello Blender Community,  I am attempting to design an object that falls onto a surface, at which point it will begin melting (via a fluid simulation at low speed and high viscosity). I want the object to fully maintain its form until it comes into contact with the surface.  As it stands, I have attempted this design with multiple layers. On layer one is the rigid body object falling onto the surface. On layer two is the exact same object with a fluid simulation applied. As it stands, I have merely switched the layers rendered during the rendering process to simulate the desired effect, but I'm not entirely satisfied with the result. Is there a better way to go about my design?

So I have a simple bouncing ball animation, but I just wanted to something "weird" for the ending. Is there a way to change the ball's material on the fly, where after the last bounce it changes materials and physical composition from a rigid body(baked animation now) to a fluid sim?  (within 30-40 frames)


It was announced today that .  The article gives the following explanation of the experiment:       When a gravitational wave passes LIGO, the tunnels deform slightly and the distance travelled by each beam changes so that they no longer cancel out.   I assume this "deformation" is space dilation, but does it also not cause time dilation? How do the effects combine to push the lasers out of phase? And why did it take until now to directly measure them?

, Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. The device measures the phase shift laser beams.  If I understand this well, the measurement assumes that a suitable gravitational wave reduces and prolongs one of the tubes and the extension will take effect in the phase-shifting laser beams.  But I think if there's a gravitational wave that passes through it, it will affect everything, even the laser beam. This means that when the wave lengthens or shortens the tube, it also lengthens or shortens the wavelength of the laser beam.  In my opinion, therefore, this device does not work. Or am I wrong?


i was working on stacks when i came across a problem where i had to double the array. i used this code which did not work but it should have.   class ArrayDouble { public static void main(String []args) {     int arr[] = new int[10];     sizeChange(arr);     for(int i=0;i<arr.length;i++)         System.out.print(arr[i]+ " "); } public static void sizeChange(int arr[]) {     arr[0]=1;     arr = new int[2*arr.length];     arr[1]=1; } }   from what i have seen, any changes that takes place in an array which has been passed as a parameter, the changes reflect back in the actual parameters right? So why doesn't the change in size of the array is reflected in the original parameter?  Also, arr[0] becomes 1 in the original array but arr[1] remains 0. why does that happen?  PS: the problem was solved when i changed the return type of sizeChange to int[] and passed arr of sizeChange and collected it into the Main arr. So i dont need the correction of the code, i just need the answer as to why this is happening.  Thank you in advanced.

I always thought Java uses pass-by-reference. However, I've seen a couple of blog posts (for example, ) that claim that it isn't (the blog post says that Java uses pass-by-value). I don't think I understand the distinction they're making. What is the explanation?


There are integers $p$ and $q$ s.t. $0 \leq q \leq p$ and $p >1$. I want to a quick way to find the smallest positive $n$ s.t. $(n \times p) \mod q = 0 $, and in addition to that, I need to find the next smallest n, and so on, indefinitely. Is there a quick way to do this without checking $n = 1,2,3,\ldots$ sequentially?

I tried to start by showing that $ \frac{a}{\gcd(a,b)} $ is always an integer, let's call it $d$, because $a$ is always a multiple of $(a,b)$ based on the definition of a g.c.d. I then tried to show that if $a | b$ then $a|bc$ from the theorem that states that $a|b$ implies that $a|bc$ for any integer $c$, but I cant seem to prove that $a | b$ or that $a|bc$ only if $d \mid c$ (where $d = \frac{a}{\gcd(a,b)}$ ). Any help would be much appreciated.


I was looking for information about parameterized queries, the problem is that I cant find basic examples to follow in php.  Is this correct?  PHP   $email= $_POST["email"]; //or mysqli_real_escape_string($_POST["email"]);?   SQL    SET @email = '$email';  SELECT email FROM users WHERE email=@email   Whit this configuration would be enough to avoid sql injections?

If user input is inserted without modification into an SQL query, then the application becomes vulnerable to , like in the following example:  $unsafe_variable = $_POST['user_input'];   mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");   That's because the user can input something like value'); DROP TABLE table;--, and the query becomes:  INSERT INTO `table` (`column`) VALUES('value'); DROP TABLE table;--')   What can be done to prevent this from happening?


From the Player's Handbook (p. 202) under Casting Time > Bonus Action,      A spell cast with a bonus action is especially swift. You must use a bonus action on your turn, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of one action.   meaning that once a bonus-action-spell is cast, following that one cannot cannot cast another non-cantrip spell as an action. However, the wording above does not seem to preclude the use of a non-cantrip spell as an action BEFORE using your bonus action to cast a bonus-action spell.   Is there anywhere else in the PHB where it is clarified whether the order matters or not? In the absence of further ruling, should we accept the literal meaning above which says "cannot cast a non-cantrip spell AFTER a bonus-action spell"?

When casting a spell as a bonus action, you can't cast another spell on the same turn except a 1-action cantrip - can the order be "reversed"? Can you cast a cantrip first as a bonus action, then any other spell? If not, would it be broken to allow it?  I'm thinking a Sorcerer casting a Quickened true strike, then casting a leveled spell requiring a touch attack. Seems like a great use of the cantrip + Metamagic feature, but it seems like it is not allowed.


I've got two commands, the first one is rhythmbox-client --print-playing, which prints (returns) a string into the terminal, and another one, notify-send text, which sends a system notification. Now I'm trying to write something like notify-send | rhythmbox-client --print-playing or notify-send ${rhythmbox-client --print-playing}, but I must admit that I'm not really comfortable with bash scripting...  How do I write this ?

How can I pipe grep output as an argument to the cd command?  For example:  [root@xxx xxx]# pip install django | grep '/usr.*'   Requirement already satisfied (use --upgrade to upgrade): django in   /usr/lib64/python2.7/site-packages   Here /usr/lib64/python2.7/site-packages is highlighted and I want to pass this string to cd.


What is the formula to calculate  $$n^0 + n^1 + n^2 + n^3 + \cdots + n^k$$  Calculate example  $$185^0 + 185^1 + 185^2 + \cdots + 185^{13}$$  Any hints appreciated!

this is very easy I think but I don't have much experience with geometric series. I need to find/simplify the sum  $x^k+x^{k+1}+\dots+x^{k+m}$   where $x\in\mathbb{R}, k\in\mathbb{Z},m\in\mathbb{N}$  So I'm thinking I just write this as  $x^k+x\times x^k+x^2\times x^k+\dots+x^m\times x^k$  Now isn't this a geometric series with $a=x^k$ and $r=x$? So it should have the sum  $$s=\frac{x^k(1-x^{k+m})}{1-x}=\frac{x^k-x^{2k+m}}{1-x}$$  Is this the correct answer?


A suggested edit came up in the queue that I wasn't sure how to handle:   (The link to the edit suggestion may or may not work for other users)  Although it is true it is unnecessary for the question, should it be removed? I guess the real question is, is Salaamun Alekum considered to be "religious content" on StackExchange, or is it more akin to a new user writing (which happens rather frequently):     Good morning. My C# ExecuteSqlCommand is not returning output from stored procedure.

I edit a lot of posts every day. I often run across posts with 'Hi' and 'Thanks' on the top and the bottom of the post respectively. I also run across things like:     --User   Should these items be removed during the editing of the post by an editor?


I bumped into the problem that I can't download any updates or new packages. I had this problem before  which I could solve through several steps  sudo rm /var/lib/apt/lists/* -vf sudo apt-get clean sudo apt-get update   This gives me still a  W: Failed to fetch http://ucmirror.canterbury.ac.nz/ubuntu/dists/quantal-security/multiverse/binary-i386/Packages 404  Not Found  E: Some index files failed to download. They have been ignored, or old ones used instead.   I tried a different server by hand, and get a 'No suitable download server found' when I ask Ubuntu to find one for me. I am not behind a firewall, there is no proxy server set up in the settings or anything like that.

Recently I have installed an older version of Ubuntu on my old machine. Whenever I try to install any software, I get an error saying it couldn't be found:  $ sudo apt-get install vlc Reading package lists... Done                Building dependency tree        Reading state information... Done     E: Couldn't find package vlc


i want to ask a question for a bedsize and while the answer is not what i choose it will be i want that it will ask the user to answer again  import java.util.Scanner;  public static void main(String[] args) {     String newBedType ;     Scanner sc1 = new Scanner(System.in) ;     System.out.println("you want a single bed or doublebed? ") ;     newBedType = sc1.next() ;      while (newBedType != "single" + "doublebed") {     System.out.println("please choose againe the bed size: ");     newBedType = sc1.next() ;     switch (newBedType) {     case "single" : System.out.println("i see you like sleeping alone");      break ;     case "doublebed" : System.out.println("got company ;) ");     break ;     }      }                 }   }  the code kinda works it shows the cases if i write the correct string but it will continue to ask me forever.....  i just stared learning java so be easy on me i know its a stupid question but after hours of trying and searching here(though i did found in python but dont know how to "translate" it to java) i cant figure it out... thanks to anyone willing to help :)

I've been using the == operator in my program to compare all my strings so far. However, I ran into a bug, changed one of them into .equals() instead, and it fixed the bug.  Is == bad? When should it and should it not be used? What's the difference?


The Polya urn model for contagion is as follow.  We start with an urn which contains one white ball and one black ball. At each second we choose a ball at random from the urn and replace it together with one more ball of the same colour.  Calculate the probability that when $n$ balls are in the urn, $i$ of them are white.  How do you prove this probability is equal to $1/(n-1)$?

Suppose we have an urn with one red ball and one blue ball. At each step, we take out a single ball from the urn and note its color; we then put that ball back into the urn, along with an additional ball of the same color.   This is Polya's urn, and one of the basic facts about it is the following: the number of red balls after $n$ draws is uniform over $\{1, \ldots, n+1\}$.   This is very surprising to me. While its not hard to show this by direct calculation, I wonder if anyone can give an intuitive/heuristic explanation why this distribution should be uniform.   There are quite a few questions on Polya's urn on math.stackexchange, but none of them seem to be asking exactly this. The closest is , where there are some nice explanations for why, assuming as above that we start with one red and one blue ball, the probability of drawing a red ball at the $k$'th step is $1/2$ for every $k$ (it follows by symmetry).


So, I have this textbook in combinatorics as part of my course in discrete math. Now, I'm not sure if this is a thing with all combinatorics textbooks, but it feels more like an advanced exercise book with some refreshers than a textbook meant for someone not very familiar with the field. Like, something you'd use to prepare for an exam or just kill time if you're already good at the subject, rather than using it to study from the ground up. Basically, they explain something, provide a couple of easy exercises, and the difficulty then abruptly picks up and oftentimes they'll suddenly get the jump on you with solutions that have nothing to do with the material taught in the section. This is all nice and good, but it makes it impossible for me to actually get the hang of anything. I think part of the reason is that the book is about 200 pages long and 50 pages are dedicated to detailed solutions, which are nice, but not as good as just more exercises. I'm used to books that provide 50+ exercises at the end of each section, starting off easy and then gradually picking up in complexity until the questions are even harder than what you'd expect in an exam. Just to emphasize the importance of this - Calculus was a complete nightmare for me at first but the book I'm studying from gradually got me on good terms with the subject, and I eventually started to enjoy it so much that I kept on studying from it without realizing that I'm well into calculus 2 material, integrating left and right, and really looking forward to the calculus 3 stuff. Now, I tried to google some recommendation, but every book I came across had one of two problems: it's either the same as the book I'm using right now (not enough exercises, so the difficulty picks up abruptly), or it has no solutions whatsoever to any of the problems. I would greatly appreciate a good recommendation. I really do appreciate this field but I can't find any entry point that isn't a frustrating mess.

I am an undergrad, math major, and I had basic combinatorics class before (undergrad level.) Currently reading Stanley's Enumerative Combinatorics with other math folks. We have found this book somewhat challenging~ Do you have any suggestions on other books to read? or books to help going thru Enumerative combinatorics?


How does cooling a magnet allow it to quantum lock/levitate? I have seen it in videos but do not know how it works.

I just saw  demonstration by someone from a Tel Aviv University lab.  What they achieved there is mind blowing. I myself own a levitron that uses the Hall effect to levitate a magnet, the problem with that is the magnet must always be flat facing the Hall effect base, any unbalance will wreck the levitation. But in the case of the video you can see that he isn't constrained by such things, the disk can levitate and stay still at whatever angle and orientation. Instead of being levitated, the disk is simply being held still.   How does this happen? He referred to this as quantum trapping, but I've little idea about what that is....  Also, what material is the base in these demos? Is the circular track in the demonstration magnetic? I assume this is so. Is there any conceivable way for this technology to make objects levitate on any surface?


I know  we can say:  The plane takes off at 6:40 p.m.(expressing future)  But can I say:  The plane takes off at 6:40 p.m. when everything is ready.  Or I should put "will" here  The plane will take off at 6:40p.m. when everything is ready.  Thanks so much!

Which one is grammatically  correct?      I don't know when he leaves for New York tomorrow.   or      I don't know when he will leave for New York tomorrow   Is it correct to use the future form of the verb leave and the  word  tomorrow in one sentence?


We are having Product , Pricebookentry and pricebook in SFDC as a standard object. here pricebookentery is having look up to product and pricebook.  so i want to get all the prodcuct fields which is associated with pricebook.  public class test123{          Public test123(){         List<PricebookEntry> pb = [select id, product2.name,Unitprice from Pricebookentry where pricebook2.name = 'Test'  ];         system.debug('@@@@' +pb);      }    List<PricebookEntry> pb = [select id, product2.name,Unitprice from Pricebookentry where pricebook2.name = 'Test'  ];    this query is working in developer console getting all the values of priceboook and products. if you call from apex class its not working please see the log below .. please help to understand Not- pricebook and prodcut is not having any relationship.         DEBUG|@@@@(PricebookEntry:{Id=01u28000007nkSJAAY,   Product2Id=01t28000001Fq7gAAC, UnitPrice=5000.00,   Pricebook2Id=01s2800000DF6PtAAL},   PricebookEntry:{Id=01u28000007nkSKAAY, Product2Id=01t28000001Fq7ZAAS,   UnitPrice=2300.00, Pricebook2Id=01s2800000DF6PtAAL},   PricebookEntry:{Id=01u28000007nkSLAAY, Product2Id=01t28000001Fq7YAAS,   UnitPrice=212.00, Pricebook2Id=01s2800000DF6PtAAL},   PricebookEntry:{Id=01u28000007nkSMAAY, Product2Id=01t28000001Fq7nAAC,   UnitPrice=1236.00, Pricebook2Id=01s2800000DF6PtAAL}

I run the test class and try to capture the permission set name in debug. But i got Id than name. Any reason why I am getting this ? Please help me to understand.  See the image, what i query in return what i got !


I need to move all my pdf files from one folder to another folder. How can I do this in terminal?

I am new in Linux so here is my very basic question. I have a directory with 41 folders. I need to create a loop so that the process goes into each folder, copy all the files and then paste everything in a specific folder.   Any idea how to approach this? It would be a lot of clicking without a code solution  In the image you can see all the folders I need to access and the output folder where I want to paste everything (zPASTE_EVERYTHING_HERE)    My path is: /shared/home/data/output  Would like to paste everything in: /shared/home/data/output/zPASTE_EVERYTHING_HERE


I want to create a chart with two different scalings (steps of 500 and steps of 10.000) on the y-axis.  This is needed because there is a fixed cost of 1000 (so a straight line from left to right) and three graphs for different scenarios, which all have rather high values in the end.   Currently this leads to the fixed cost line being extremely small and barely visible, because I have to choose a rather big scale (10.000) in order to get a reasonably sized chart.   However, I want this fixed cost line and especially the points of contact clearly visible.   Imho, to achieve that I need to use two different scales. I came up with a quick and dirty sketch of my desired result in MS Paint:    Is it possible to achieve this with Excel? Appreciate any tips

I want to break the Y-axis in an Excel sheet. How can I do that?  For example, suppose I have the data in a range from 0-7000 and then range from 22000-28000. I want a break in the Y axis from 8000-20000.


Let $G$ denote the group of all the automorphisms of the field $F_{3^{100}}$ that consists of $3^{100}$ elements .How to count the number of distinct subgroups of $G$ ?  I have not gathered enough points to solve it.So cant give my thoughts here.Any hints will be useful

Let $G$ denote the group of all the automorphisms of the field $\mathbf{F}_{3^{100}}$ that consists of $3^{100}$ elements. What is the number of distinct subgroups of $G$?


I am getting the error:   touch: cannot touch `/opt/tsrm/compliance/cme/log/20121207.log`: No such file or directory   on the touch command: touch $LOGFILE  I also checked the link: , But I didn't understand the answer.   Note: I was also getting mkdir: cannot create directory; I fixed this by adding the -p option.   Could this be something with the version of Linux I am working in?

What could cause touch to fail with this error message?  touch: cannot touch `foo': No such file or directory   Note that an error due to incorrect permissions looks different:  touch: cannot touch `foo': Permission denied


I use Ubuntu 18.04 with Gnome Shell and gdm3.  When I press Ctrl + Alt + F1 I see gdm3 home screen. My current desktop session is on the second "screen" Ctrl + Alt + F2.  Is it a bug?  I had a random bug and I don't know if it's related to the duplicate gdm3 instances: when I logged back after locking my screen, I was unable to reopen with session and I had to reboot the computer by pressing Ctrl + Alt + Del on another screen, like Ctrl + Alt + F3.  I removed gdm3 with sudo apt remove --purge gdm3 and reinstalled it and there are still 2 instances.  Update: once my screen is locked, sometimes I can't log in again as my password is rejected, but if I switch to the first "screen" with Ctrl + Alt + F1 I can log in again.

I am running Ubuntu Gnome 16.10 (64bit). Gnome-shell version 3.20.4.   When I start my computer, I eventually see GDM3 offering a login prompt, as expected.   After I login, my monitors flash a little (like they're unplugged and then replugged in), and then I am brought to the gnome 3 desktop.   However, if I hit  CTRL + ALT + F1, I am brought back to GDM3, offering a login prompt.   If I then hit CTRL + ALT + F2,  I am brought back to my Gnome 3 desktop.  So it seems like GDM3 continues to run in TTY1 while the desktop runs in TTY2.   Questions   Is this how things are supposed to work? I thought the desktop was supposed to use the same TTY (or whatever you call it) as the display manager.  Am I using more graphical resources by having things setup this way? Is there a way to have GDM3 pass off the display to Gnome Desktop after I login? Is there any advantage do doing #3?


I know there are a lot of questions about very long suspension periods, but I came across a case that is quite perplexing. This user had many offenses to earn his long ban, despite being a very prolific and high-rep user with many valuable posts, but the suspension period assigned strains the definition of "suspension."     This account is temporarily suspended network-wide. The suspension   period ends on Mar 18 '(22)92 at 16:28.   I can see why you might want to suspend an account for several years, maybe even a decade, if you don't believe behavior will change. However, what is the point of "suspending" an account for 275 years? Is there a reason why you can't just make it a permanent ban instead of pretending it is a suspension?    On a tangential note, isn't the "temporary" in "temporary suspension" redundant?  Suspend: temporarily prevent from continuing or being in force or effect.

I ran into a user profile that states:  This account is  to cool down. The suspension period ends on Apr 27 '24 at 7:54  At first, it seemed like a somewhat longish suspension period.  Referring to  reveals:  Depending on the severity of the problem behavior — and at the complete discretion of the moderator — your account will be placed in timed suspension for anywhere from 1 to 365 days.  This indicates that temporary suspensions can last upto a year.  Additionally, the  doesn't give any other information about the timed suspension other than the link to the wiki above. I am keen to know whether the example above (wherein the account has been suspended until 2024) is a one-off case or is it a case of documentation being obsolete?


I'd like to pipe a process to less, and then issue a command to the scroll the end of the current piped text. That is, shift+G blocks if the command hasn't yet closed its stdout, so for a long-running command you can't see the end until it finishes.  As an example:  i=0; while true; i=$((i+1)); do echo $i; sleep 0.001; done | less   If you hit shift+G once less starts paging, it doesn't scroll down and just blocks indefinitely.  Note that I'm not worried about the buffering - the output is frequent enough that the buffer doesn't cause much delay. Rather, I'd like less not to hang when scrolling to the end of the current buffer (note that it hangs before updating the display, so I do not even see the end of the buffer).

I have a script which calls two commands:  long_running_command | print_progress   The long_running_command prints progress but I'm unhappy with it. I'm using print_progress to make it nicer (namely, I print the progress in a single line).  The problem: Connection a pipe to stdout also activates a 4K buffer, so the nice print program gets nothing ... nothing ... nothing ... a whole lot ... :)  How can I disable the 4K buffer for the long_running_command (no, I do not have the source)?


This is a follow-up to , specifically to .  I understand the concern of wreaking havoc on the duration of spells and other effects by delaying your turn at will in the initiative order, hence why it's not possible to "change your place" once you are engaged in an encounter.  However, in a game I played yesterday, something happened that made me think: my character was sleeping at the start of an encounter (a night ambush), and so the DM only made me roll initiative when another character woke mine up.  What I'm wondering is whether it's possible for a creature to willingly delay entering an encounter that they can see, in order to roll initiative later than the other involved creatures, thus not changing its initiative position, just being added later. Is such a thing possible per RAW?  Technically speaking, it's not that different from seeing a fight between two distant NPCs and choosing not to get involved right away, but instead picking the right moment to do so (and rolling initiative at that point). But contrary to  (where the character just arrives later and rolls initiative right when he sees the encounter), I specifically want to know whether you can see an encounter but choose not to roll initiative just yet, essentially delaying your participation in the encounter until a later time.

I am DMing a small hunger-games-like adventure where there are multiple parties of NPC who also are participating.  It is pretty likely that the PCs will happen upon some of the NPCs fighting some monsters. How do I handle the PCs joining the fight, do I roll initiative all over again? Do I just roll for the new combatants and add them to the round?  Also see this similar question for Pathfinder:


difference between try...catch and try....finally ? in asp.net(C#)  like when i want to catch error like 1/0 then i put code in try block and put exception object in catch block like response.write("ERROR:"+ ex.Message) but advisors told me that it isn't a good practice to put catch always, it absorbs error without notifying ????? ehhhhh ? but it did via ex.Message , so why ?  and what does try....finally do ? i know that it is used to release resources but of what use is TRY if exception can't be catched ?

I have seen people say that it is bad form to use catch with no arguments, especially if that catch doesn't do anything:  StreamReader reader=new  StreamReader("myfile.txt"); try {   int i = 5 / 0; } catch   // No args, so it will catch any exception {} reader.Close();   However, this is considered good form:  StreamReader reader=new  StreamReader("myfile.txt"); try {   int i = 5 / 0; } finally   // Will execute despite any exception {   reader.Close(); }   As far as I can tell, the only difference between putting cleanup code in a finally block and putting cleanup code after the try..catch blocks is if you have return statements in your try block (in that case, the cleanup code in finally will run, but code after the try..catch will not).  Otherwise, what's so special about finally?