diff --git a/math/mat4x4.hpp b/math/mat4x4.hpp
index 1a5c25e..ca8a989 100644
--- a/math/mat4x4.hpp
+++ b/math/mat4x4.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <utility>
+#include "vec4.hpp"
 
 template <int R, int C, typename T>
 struct mat;
diff --git a/math/vec3.hpp b/math/vec3.hpp
index 7383cee..c05dd72 100644
--- a/math/vec3.hpp
+++ b/math/vec3.hpp
@@ -147,3 +147,11 @@ inline constexpr T length(vec<3, T> const& v)
 {
   return sqrt(dot(v, v));
 }
+
+template<typename T>
+inline constexpr vec<3, T> cross(vec<3, T> const& x, vec<3, T> const& y)
+{
+  return vec<3, T>(x.y * y.z - y.y * x.z,
+		   x.z * y.x - y.z * x.x,
+		   x.x * y.y - y.x * x.y);
+}