Following code not working in LaTeX : $\{f^'_n\}$

Question

Why does the following code does'nt work when i am using PDFLaTeX or just LaTeX ? I tried it in different editors as well.

This is the code

 $\{f^'_n\}$

Surprisingly it works on Math.SE site

Answer 1

In math mode, ' is active and is defined as ^\prime. Thus you should not use ^': just ' will work:

\documentclass{article}
\begin{document}
$\{f'_n\}$
\end{document}