array in align environment

Question

LaTex produces the formula I want:

but it returns some warning messages. The is the code of the formula above, I don't know where is the mistake:

\begin{align}
\mathbb{D} (k,n) = \frac{2}{n_q} ~\text{cos} \bigg(\frac{(n-1)(k-1)\pi}{n_q /2} \bigg) \cdot 
\left\{ \begin{array}{c l l}
1/2 &  \mbox{if} & n=\{1, n_q /2+1 \}\\
1   & \multicolumn{2}{l}{\mbox{otherwise}} \\
\end{array} \right
\end{align}

It should be \right. (with a period after \right). Also it should be \cos and not \text{cos}. — egreg, Sep 18 '17 at 12:40
Use \cos instead of \text{cos} and \text instead of \mbox — , Sep 18 '17 at 12:44

score 0 · Accepted Answer · answered Sep 18 '17 at 12:46

\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}

array in align environment

1 Answers1