diff --git a/ex_01_03.py b/ex_01_03.py
index d0e1b46..2c56197 100644
--- a/ex_01_03.py
+++ b/ex_01_03.py
@@ -3,6 +3,9 @@
 import math
 
 def bailey_borwein_plouffe_pi(n):
+	"""
+	See https://en.wikipedia.org/wiki/Approximations_of_%CF%80#Efficient_methods
+	"""
 	result = 0
 	for k in range(n):
 		result += (1/16)**k * ( 4/(8*k + 1) - 2/(8*k + 4) - 1/(8*k + 5) - 1/(8*k + 6))