title: LaTeX undefined control sequence
body: <p>I'm working on some homework, and I was checking to make sure my code would compile to a PDF. I continually get an error that reads :</p>
  
  <pre><code>! Undefined control sequence.
  &lt;recently read&gt; \urcorner
  l.74 (($\urcorner
  $
  The control sequence at the end of the top line
  of your error message was never \def'ed. If you have
  misspelled it (e.g., `\hobx'), type `I' and the correct
  spelling (e.g., `I\hbox'). Otherwise just continue,
  and I'll forget about whatever was undefined.
  </code></pre>
  
  <p>Here is my code:</p>
  
  <p><a href="https://i.stack.imgur.com/t3lW5.png" rel="nofollow noreferrer" title="Click to enlarge"><img src="https://i.stack.imgur.com/t3lW5.png" alt="enter image description here" title="Click to enlarge"></a></p>
  
  <p>Here is my preamble:</p>
  
  <p><a href="https://i.stack.imgur.com/uFa8B.png" rel="nofollow noreferrer" title="Click to enlarge"><img src="https://i.stack.imgur.com/uFa8B.png" alt="enter image description here" title="Click to enlarge"></a></p>