diff --git a/lib/extramath.js b/lib/extramath.js
index 96f256a..f0350cd 100644
--- a/lib/extramath.js
+++ b/lib/extramath.js
@@ -27,6 +27,7 @@ function DTM() {
     };
 }
 
+// Cosine similarity: https://en.wikipedia.org/wiki/Cosine_similarity
 function cos(A, B) {
     var dotproduct = 0;
     var mA = 0;