diff --git a/cpp/src/math.hpp b/cpp/src/math.hpp
index 4068b7e..f6ac078 100644
--- a/cpp/src/math.hpp
+++ b/cpp/src/math.hpp
@@ -9,6 +9,7 @@
 #include <numeric>
 #include <ranges>
 #include <utility>
+#include <functional>
 
 template <typename T>
   requires std::floating_point<T>