LaTeX Forum

\documentclass[ngerman,parskip=half-,twoside]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage{babel}

\usepackage[allowspaces,fixamsmath,tbtags]{mathtools}
\newtagform{plain}{}{}
\usepackage{cases}

\begin{document}
  \bgroup
    \usetagform{plain}
    \begin{numcases}{\biggl( \frac{2}{p} \biggr) = \Bigl( {-1}^{\frac{p^2-1}{8}} \Bigr) = }
      +1, & falls $p\equiv \pm 1 \pmod 8$ \notag \\
      -1, & falls $p\equiv \pm 3 \pmod 8$ \tag{$\diamond$}  
    \end{numcases}
  \egroup
\end{document}

\documentclass[ngerman,parskip=half-,twoside]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage{babel}

\usepackage[allowspaces,fixamsmath,tbtags]{mathtools}
\newtagform{plain}{}{}
\usepackage{cases}

\begin{document}
  \bgroup
    \usetagform{plain}
    \renewcommand{\theequation}{$\diamond$}
    \begin{numcases}{\biggl( \frac{2}{p} \biggr) = \Bigl( {-1}^{\frac{p^2-1}{8}} \Bigr) = }
      +1, & falls $p\equiv \pm 1 \pmod 8$ \notag \\
      -1, & falls $p\equiv \pm 3 \pmod 8$  
    \end{numcases}
  \egroup
\end{document}

\documentclass[ngerman, fontsize=11pt, DIV=15, BCOR = 10mm, parskip=half-, twoside]{scrartcl}

\usepackage[backend=biber, bibencoding=utf8, style = alphabetic, natbib=true,  urldateusetime=true]{biblatex}
\usepackage{babel}% sonst ist Option ngerman bei \documentclass sinnlos
\usepackage{amsthm}
\usepackage{amsmath}

\usepackage{cases}

\begin{document}

\renewcommand{\theequation}{$\diamond$}
\begin{numcases}{(\frac{2}{p}) = ({-1}^{\frac{p^2-1}{8}) = }}
+1, &falls $p\equiv \pm 1 \pmod 8$ \nonumber \\
-1, &falls $ p\equiv \pm 3 \pmod 8$  
\end{numcases}

\end{document}