diff --git a/chapter09.tex b/chapter09.tex
index fd2948f..572e6aa 100644
--- a/chapter09.tex
+++ b/chapter09.tex
@@ -1314,6 +1314,8 @@ retrieve single values.
 We focus on an operation that increases all
 elements in a range $[a,b]$ by $x$.
 
+\index{difference array}
+
 Surprisingly, we can use the data structures
 presented in this chapter also in this situation.
 To do this, we build a \key{difference array}