IsotropicTensor: Avoid unnecessary arithmetic

dune/fufem/mechanics/isotropictensor.hh

@@ -6,18 +6,19 @@
 template <int dim, typename field_type = double>
 class IsotropicTensor : public ElasticityTensor<dim, field_type> {
 public:
-  IsotropicTensor(field_type E, field_type nu) : E_(E), nu_(nu) {}
+  IsotropicTensor(field_type E, field_type nu)
+      : twoMu_(E / (1 + nu)), lambda_(nu * twoMu_ / (1 - 2 * nu)) {}
 
   void virtual mv(SymmetricTensor<dim> const &x,
                   SymmetricTensor<dim> &y) const override {
     y = x;
-    y *= E_ / (1 + nu_);
-    y.addToDiag(E_ * nu_ / (1.0 + nu_) / (1.0 - 2.0 * nu_) * x.trace());
+    y *= twoMu_;
+    y.addToDiag(lambda_ * x.trace());
   }
 
 private:
-  field_type const E_;
-  field_type const nu_;
+  field_type const twoMu_;
+  field_type const lambda_;
 };
 
 #endif