diff --git a/bench/cec2020/benchFunctions.go b/bench/cec2020/benchFunctions.go
index 3d301d6..ecc6d41 100644
--- a/bench/cec2020/benchFunctions.go
+++ b/bench/cec2020/benchFunctions.go
@@ -48,6 +48,7 @@ func Schwefel(x []float64) float64 {
 }
 
 // LunacekBiRastrigin is the "Shifted and Rotated Lunacek bi-Rastrigin Function" of CEC2020.
+// ref: https://al-roomi.org/benchmarks/unconstrained/n-dimensions/229-lunacek-s-bi-rastrigin-function.
 func LunacekBiRastrigin(x []float64) float64 {
 	var sum0 float64