diff --git a/chapter23.tex b/chapter23.tex
index 82d27f6..5b5d0e1 100644
--- a/chapter23.tex
+++ b/chapter23.tex
@@ -749,7 +749,8 @@ $2 \rightarrow 1 \rightarrow 4 \rightarrow 2 \rightarrow 5$.
 \index{Kirchhoff's theorem}
 \index{spanning tree}
 
-\key{Kirchhoff's theorem} provides a way
+\key{Kirchhoff's theorem}\footnote{G. R. Kirchhoff (1824--1887) was
+a German physicist.} provides a way
 to calculate the number of spanning trees
 of a graph as a determinant of a special matrix.
 For example, the graph