\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\sigma = \frac{n \alpha_1 k p_e^{n-1}}{(1 + k p_e^n)^2}
\end{equation*}
\end{document}