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: 
Use `\mathop{}`:

```
\begin{equation}
\mathop{maximize}_{\textbf{P}}
\end{equation}

```
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)