Question: How can I write a letter under a word. Eg: Assume that I have to write `$\textbf{P}$` centered under the word "maximize". When I tried with "\_", the output is 

```
\begin{equation}
maximize_{\textbf{P}}
\end{equation}

```

Answer: 
You can use `\DeclareMathOperator`, from the `amsmath` package; this will give you the proper font and proper spacing. Since your operator should have subscripts placed in "limits" as with `\lim`, you need to use the starred form `\DeclareMathOperator*`. An example:

```
\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator*{\maxi}{maximize}

\begin{document}

in-line math: $\maxi_{\mathbf{P}}$

displayed expression:
\[
\maxi_{\mathbf{P}}
\]

\end{document}

```

![enter image description here](https://i.stack.imgur.com/G5rgI.png)
The just-submitted `stackengine` package is set up to do this (Being just submitted, CTAN has not yet installed it). Nice thing is you can specify the undergap as an optional argument, as shown in this example. If you want the arguments set in math-mode format, enclose them in math delimiters.

```
\documentclass{article}
\usepackage{stackengine}
\begin{document}
\(\stackunder{maximize}{\textbf{P}}\)
\(\stackunder[5pt]{maximize}{\textbf{P}}\)
\end{document}

```

![enter image description here](https://i.stack.imgur.com/r9nLN.jpg)