diff --git a/pandas_eval.py b/pandas_eval.py
index 73524c3..ce31795 100644
--- a/pandas_eval.py
+++ b/pandas_eval.py
@@ -12,6 +12,9 @@ import matplotlib.pyplot as plt
 # Pandas
 # https://pandas.pydata.org/pandas-docs/stable/index.html
 
+# Knowledge
+# https://www.investopedia.com/articles/technical/02/050602.asp
+
 def sin(f, a, N):
 	result = np.empty(0)
 	for n in range(0, N):