diff --git a/numerical_methods/ordinary_least_squares_regressor.cpp b/numerical_methods/ordinary_least_squares_regressor.cpp
index 4761a6d96..7482d84da 100644
--- a/numerical_methods/ordinary_least_squares_regressor.cpp
+++ b/numerical_methods/ordinary_least_squares_regressor.cpp
@@ -10,7 +10,7 @@
  * \author [Krishna Vedala](https://github.com/kvedala)
  */
 #include <cassert>
-#include <cstdlib>  // for std::abs
+#include <cmath>    // for std::abs
 #include <iomanip>  // for print formatting
 #include <iostream>
 #include <vector>