答案1
使用另一个alignedat
。但文字比符号更清晰。
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\section{How you want it}
\begin{equation*}
\begin{alignedat}{2}
& f(x) && =
\begin{cases}
\begin{alignedat}{2}
-1, & \quad & -\pi & < x < 0 \\
1, & \quad & 0 & < x < \pi
\end{alignedat}
\end{cases}
\\[1ex]
& f(x+2\pi) &&= f(x)
\end{alignedat}
\end{equation*}
\section{How I'd do it}
Let $f$ be the function defined over the reals, except at
integral multiples of $\pi$, given by
\begin{equation*}
f(x) =
\begin{cases}
\begin{alignedat}{2}
-1, & \quad & -\pi & < x < 0 \\
1, & \quad & 0 & < x < \pi
\end{alignedat}
\end{cases}
\end{equation*}
and extended by $2\pi$~periodicity.
\end{document}
或者,中间:两个等号实际上彼此没有关联,并且顶行中的空白很糟糕。
\begin{equation*}
\begin{gathered}
f(x)=
\begin{cases}
\begin{alignedat}{2}
-1, & \quad & -\pi & < x < 0 \\
1, & \quad & 0 & < x < \pi
\end{alignedat}
\end{cases}
\\[1ex]
f(x+2\pi) = f(x)
\end{gathered}
\end{equation*}