diff --git a/chapter01.tex b/chapter01.tex
index ea03f06..e95308c 100644
--- a/chapter01.tex
+++ b/chapter01.tex
@@ -525,7 +525,9 @@ Each sum of the form
 where $k$ is a positive integer,
 has a closed-form formula that is a
 polynomial of degree $k+1$.
-For example,
+For example\footnote{\index{Faulhaber's formula}
+There is even a general formula for such sums, called \key{Faulhaber's formula},
+but it is too complex to be presented here.},
 \[\sum_{x=1}^n x = 1+2+3+\ldots+n = \frac{n(n+1)}{2}\]
 and
 \[\sum_{x=1}^n x^2 = 1^2+2^2+3^2+\ldots+n^2 = \frac{n(n+1)(2n+1)}{6}.\]