From 5bceea2b478cb462f0b474395d0df2b651aec355 Mon Sep 17 00:00:00 2001
From: unlockable <g@unlockableworld.com>
Date: Sun, 26 Mar 2023 23:15:55 +0800
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---
 13多变量函数的连续性.tex | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/13多变量函数的连续性.tex b/13多变量函数的连续性.tex
index 3df984f..28565db 100644
--- a/13多变量函数的连续性.tex
+++ b/13多变量函数的连续性.tex
@@ -1,5 +1,11 @@
 \chapter{多变量函数的连续性}
 \section{欧氏空间}
+\begin{definition}[集合的积集]
+    设$A, B$是两个集合，定义
+    \[A \times B = \{(a, b) \mid a \in A, b \in B\}\]
+    并称之为$A$同$B$的积集。将$A \times A$简记为$A^2$。
+\end{definition}
+
 \begin{definition}[$n$维向量空间]
     我们定义
     \[\ndreal = \{(x_1, x_2, \dots, x_n) \mid x_i \in \realnum, i = 1, 2, \dots, n\}\]