diff --git a/math/magic_number.cpp b/math/magic_number.cpp
index 4a6edac4f..dd5cdc28f 100644
--- a/math/magic_number.cpp
+++ b/math/magic_number.cpp
@@ -36,7 +36,7 @@ bool magic_number(int n) {
 /** Test function
  * @returns void
  */
-void tests() {
+static void tests() {
     std::cout << "Test 1:\t n=60\n";
     assert(magic_number(60) == false);
     std::cout << "passed\n";