diff --git a/luku18.tex b/luku18.tex
index 3500a38..242c1a5 100644
--- a/luku18.tex
+++ b/luku18.tex
@@ -87,7 +87,7 @@ as a sum where each term is a power of two.
 
 \index{node array}
 
-A \key{node array} contains the nodes of a rooted tree
+A \key{node array}\footnote{A similar idea is sometimes called the \key{Euler tour technique} \cite{tar84}.} contains the nodes of a rooted tree
 in the order in which a depth-first search
 from the root node visits them.
 For example, in the tree