diff --git a/u02/src/tests.cpp b/u02/src/tests.cpp
index 90dda1e..9e224dd 100644
--- a/u02/src/tests.cpp
+++ b/u02/src/tests.cpp
@@ -330,6 +330,9 @@ TEST_CASE("Bresenham circle (prop: √(x²+y²)-r<ε)") {
   //
   // When the points are rounded to the nearest integer,
   // M must be rounded down and m rounded down.
+  //
+  // An interactive version of this can be found here:
+  // https://www.desmos.com/calculator/kn19qhue20
   const int size = 100;                                         // s
   const int max_c = std::floor(size / (3 - std::sqrt(2)));      // M (2)
   const int min_c = std::ceil(((4 - std::sqrt(2)) / 7) * size); // m (3)