diff --git a/2_critical_filter.py b/2_critical_filter.py
index ba7e37bfc2dcb46b869fefc9fa4bb652070d9c7d..eb53f0bd82e86b4330e3acaf025e656e9dcd607a 100644
--- a/2_critical_filter.py
+++ b/2_critical_filter.py
@@ -1,3 +1,20 @@
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# Copyright(C) 2013-2019 Max-Planck-Society
+#
+# NIFTy is being developed at the Max-Planck-Institut fuer Astrophysik.
+
 import numpy as np
 
 import nifty5 as ift