Text subscript within math environment ($$)

Question

How can I make a subscript of 'p' in fp(x) = ...

I have this code right now, but I think there's a better way:

$f\textsc{$p$}(x)=x^3-x^2+px+1$

Answer 1

Subscripts in math mode are initiated using _, as in the following example:

\documentclass{article}
\begin{document}
$f_p(x) = x^3 - x^2 + px + 1$
\end{document}

For sub-scripting multiple items, brace them using _{...}. If you want to write text in the subscript, use amsmath's \text for the prevailing text font, or \mathrm for a forced upright font:

\documentclass{article}
\usepackage{amsmath}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}
This is a theorem
\begin{align*}
  f_{\text{prob}}(x)   &= x^3 - x^2 + px + 1 \\
  f_{\mathrm{prob}}(x) &= x^3 - x^2 + px + 1
\end{align*}
\end{theorem}
Some regular text.
\begin{align*}
  f_{\text{prob}}(x)   &= x^3 - x^2 + px + 1 \\
  f_{\mathrm{prob}}(x) &= x^3 - x^2 + px + 1
\end{align*}
\end{document}