diff --git a/triqs/gfs/domains/matsubara.hpp b/triqs/gfs/domains/matsubara.hpp
index 5959a649..cc8fcb81 100644
--- a/triqs/gfs/domains/matsubara.hpp
+++ b/triqs/gfs/domains/matsubara.hpp
@@ -21,6 +21,7 @@
 #pragma once
 #include "../tools.hpp"
 #include <triqs/utility/arithmetic_ops_by_cast.hpp>
+#include <triqs/utility/kronecker.hpp>
 
 namespace triqs {
 namespace gfs {
@@ -103,6 +104,11 @@ namespace gfs {
 
  using matsubara_freq_domain = matsubara_domain<true>;
  using matsubara_time_domain = matsubara_domain<false>;
+
+
+ // ----- kronecker function : overload for matsubara_freq 
+ inline bool kronecker(matsubara_freq const & freq) { return freq.n == 0; }
+ inline bool kronecker(matsubara_freq const & f1, matsubara_freq const &f2) { return f1.n == f2.n; }
 }
 }
 
diff --git a/triqs/utility/kronecker.hpp b/triqs/utility/kronecker.hpp
new file mode 100644
index 00000000..8cb87ea7
--- /dev/null
+++ b/triqs/utility/kronecker.hpp
@@ -0,0 +1,30 @@
+/*******************************************************************************
+ *
+ * TRIQS: a Toolbox for Research in Interacting Quantum Systems
+ *
+ * Copyright (C) 2014 by O. Parcollet
+ *
+ * TRIQS 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.
+ *
+ * TRIQS 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
+ * TRIQS. If not, see <http://www.gnu.org/licenses/>.
+ *
+ ******************************************************************************/
+#pragma once
+#include <triqs/clef.hpp>
+
+inline bool kronecker(int i, int j) { return i == j; }
+
+namespace triqs {
+namespace clef {
+ TRIQS_CLEF_MAKE_FNT_LAZY(kronecker);
+}
+}