排版定理与证明

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\begin{document}
\section{Parity}
\begin{definition}
An integer is even if it equals \(2k\) for some integer \(k\).
\end{definition}
\begin{lemma}\label{lem:even-sum}
The sum of two even integers is even.
\end{lemma}
\begin{proof}[Proof of Lemma~\ref{lem:even-sum}]
Write the integers as \(2m\) and \(2n\). Since \(m+n\) is an integer,
\[
  2m+2n=2(m+n), \qedhere
\]
\end{proof}
\begin{corollary}
Any finite sum of even integers is even.
\end{corollary}
\end{document}