fix compilation of snippets
diff --git a/doc/snippets/compile_snippet.cpp.in b/doc/snippets/compile_snippet.cpp.in
index 561d5f6..4745423 100644
--- a/doc/snippets/compile_snippet.cpp.in
+++ b/doc/snippets/compile_snippet.cpp.in
@@ -4,6 +4,7 @@
 #include <Eigen/Cholesky>
 #include <Eigen/Geometry>
 #include <Eigen/Jacobi>
+#include <Eigen/Eigenvalues>
 #include <iostream>
 
 using namespace Eigen;