答案1
\documentclass[ngerman]{article}
\usepackage{array}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{fontspec}
\begin{document}
\begin{align}
\mathbb{D} (k,n) = \frac{2}{n_q} ~\cos \bigg(\frac{(n-1)(k-1)\pi}{n_q /2} \bigg)
\cdot
\begin{cases}
1/2 & \text{if } n=\{1, n_q /2+1 \}\\
1 & \text{otherwise} \\
\end{cases}
\end{align}
\end{document}