diff --git a/modules/14_cpp26/include/constexpr_frontier.hpp b/modules/14_cpp26/include/constexpr_frontier.hpp
index 7b33ebb..bbeec7f 100644
--- a/modules/14_cpp26/include/constexpr_frontier.hpp
+++ b/modules/14_cpp26/include/constexpr_frontier.hpp
@@ -1,12 +1,23 @@
 #pragma once
 
+#include <algorithm>
 #include <array>
-#include <cstddef>
 
 namespace cpp26 {
 
-[[nodiscard]] auto compile_time_factorial(int n) -> long long;
-[[nodiscard]] auto compile_time_sum(std::array<int, 5> values) -> int;
-[[nodiscard]] auto is_sorted(std::array<int, 5> values) -> bool;
+[[nodiscard]] constexpr auto compile_time_factorial(int n) -> long long {
+    auto result = 1LL;
+    for (auto i = 2; i <= n; ++i) {
+        result *= i;
+    }
+    return result;
+}
+[[nodiscard]] constexpr auto compile_time_sum(std::array<int, 5> values) -> int {
+    return std::ranges::fold_left(values, 0, std::plus{});
+}
+[[nodiscard]] constexpr auto is_sorted(const std::array<int, 5> &values) -> bool {
+    return std::ranges::is_sorted(values); // also a constexpr function
 
-}  // namespace cpp26
+}
+
+}  // namespace cpp26
\ No newline at end of file
diff --git a/modules/14_cpp26/src/constexpr_frontier.cpp b/modules/14_cpp26/src/constexpr_frontier.cpp
index d50c521..b8d7fd4 100644
--- a/modules/14_cpp26/src/constexpr_frontier.cpp
+++ b/modules/14_cpp26/src/constexpr_frontier.cpp
@@ -1,20 +1,6 @@
 #include "constexpr_frontier.hpp"
 
 namespace cpp26 {
-
-auto compile_time_factorial(int n) -> long long {
-    (void)n;
-    return 1;
-}
-
-auto compile_time_sum(std::array<int, 5> values) -> int {
-    (void)values;
-    return 0;
-}
-
-auto is_sorted(std::array<int, 5> values) -> bool {
-    (void)values;
-    return false;
-}
+    // constexpr functions should be defined in the header file
 
 }  // namespace cpp26