Update optimizer.py

Author: Evolutionary-Intelligence

pypop7/optimizers/core/optimizer.py

@@ -31,8 +31,13 @@ class Optimizer(object):
     ----------
     Holland, J.H., 1992.
     Adaptation in natural and artificial systems: An introductory analysis with
-    applications to biology, control, and artificial intelligence.
+    applications to biology, control, and artificial intelligence (Second Edition).
     MIT Press.
+
+    Rechenberg, I., 1973.
+    Evolutionsstrategie: Optimierung technischer systeme nach prinzipien der
+    biologischen evolution.
+    Frommann-Holzboog Verlag, Stuttgart.
     """
     def __init__(self, problem, options):
         # problem-related settings