Help to write a formula

Question

I want to write this equation from the image.

This is what I tried but I am getting an error about math mode only

\begin{equation}
 v_\pi(s) = \mathbb{E}(G\textsubscript{t}\mid S\textsubscript{t} = s) = 
  \mathbb{E}\textsubscript{\pi} (\sum_{n=0}^{+\infty} \gamma^k R\textsubscript{t+k+1} \mid S\textsubscript{t} = s)
  \end{equation}
  

Answer 1

As any beginner's guide to LaTeX will tell you, math subscripts are entered prefixing them with _. As the name says, \textsubscript is for subscripts in text mode (the command itself was added rather recently).

Exactly like in v_{\pi}: why should the other cases be different?

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{equation}
v_{\pi}(s)
= \mathbb{E}_{\pi}[G_{t}\mid S_{t} = s]
= \mathbb{E}_{\pi} \biggl[\,\sum_{n=0}^{+\infty} \gamma^k R_{t+k+1} \biggm| S_{t} = s\biggr]
\end{equation}

\end{document}