\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
    \mathcal{N}(\rho) := \sum_i
        \frac{|\lambda_{i}|-\lambda_{i}}{2}
\end{equation*}
\end{document}