diff --git a/chapter19.tex b/chapter19.tex
index 1949b3f..8e27242 100644
--- a/chapter19.tex
+++ b/chapter19.tex
@@ -640,9 +640,11 @@ a complete tour will be found quickly.
 \index{heuristic}
 \index{Warnsdorf's rule}
 
-\key{Warnsdorf's rule}\footnote{This heuristic was proposed
-in Warnsdorf's book \cite{war23} in 1823.} is a simple and effective heuristic
-for finding a knight's tour.
+\key{Warnsdorf's rule} is a simple and effective heuristic
+for finding a knight's tour\footnote{This heuristic was proposed
+in Warnsdorf's book \cite{war23} in 1823. There are
+also polynomial algorithms for finding knight's tours
+\cite{par97}, but they are more complicated.}.
 Using the rule, it is possible to efficiently construct a tour
 even on a large board.
 The idea is to always move the knight so that it ends up
diff --git a/list.tex b/list.tex
index 9bf6ff6..8ae33c1 100644
--- a/list.tex
+++ b/list.tex
@@ -261,6 +261,11 @@ pro  \emph{Annals of Mathematics}, 3(1/4):35--39, 1901.
   Where to use and how not to use polynomial string hashing.
   \emph{Olympiads in Informatics}, 7(1):90--100, 2013.
 
+\bibitem{par97}
+  I. Parberry.
+  An efficient algorithm for the Knight's tour problem.
+  \emph{Discrete Applied Mathematics}, 73(3):251--260, 1997.
+
 % \bibitem{pic99}
 %   G. Pick.
 %   Geometrisches zur Zahlenlehre.