I want to have '' after sum

Question

\sum_{k=0}^{n} f(x_{k})

I want to change sth in this code to get sth like this image. I have problem where put ''.

Answer 1

Just use

\sum_{k=0}^{n}{}'' f(x_{k})

Answer 2

The \sideset command from the amsmath package is probably more suitable.

\documentclass{article}
\usepackage{mathtools}   % loads »amsmath«

\begin{document}
  \[
    \sideset{}{''}\sum_{k=0}^{n} f(x_{k})
  \]
\end{document}