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\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\begin{cases}
c \colon \{1, \dots, n\} \rightarrow \{1, \dots, n\} \text{ such that}\\
c(a_i) = a_{i+1} \text{ for $1\le i<l$}\\
c(a_l) = a_1
\end{cases}
\end{equation*}
\end{document}