
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{amsmath}
\pagestyle{empty}
\begin{document}
\[
A \xleftarrow{n+\mu-1} B 
\xrightarrow[T]{n\pm i-1} C
\]
\end{document}
