
\documentclass{article}
  \usepackage[body={8cm,8cm}]{geometry}
  \usepackage{lmodern}
  \usepackage{amsmath}
  \pagestyle{empty}

\begin{document}
\[
\underset{a}{X} = 
    \sum_{k=a}^{\infty} X^k 
\]
\end{document}
