diff --git a/docs/chapter04.md b/docs/chapter04.md
index 4a5b5d0..8b905dc 100644
--- a/docs/chapter04.md
+++ b/docs/chapter04.md
@@ -78,4 +78,4 @@ $$
 \end{bmatrix}
 $$
 
-$n$阶方阵的置换矩阵有$\binom{n}{1}=n!$个。
+$n$阶方阵的置换矩阵有$P(n,n)=n!$个。