Oliver Sander
--- a/dune/istl/multitypeblockmatrix.hh
+++ b/dune/istl/multitypeblockmatrix.hh
+        });
+      });
+    }
+
+
+    //===== norms
+
+    //! square of frobenius norm, need for block recursion
+    auto frobenius_norm2 () const
+    {
+      using field_type = typename std::decay_t<decltype((*this)[Indices::_0][Indices::_0])>::field_type;
+      typename FieldTraits<field_type>::real_type sum=0;
+
+      auto rows = index_constant<N()>();
+      Hybrid::forEach(Hybrid::integralRange(rows), [&](auto&& i) {
+        auto cols = index_constant<this->M()>();