Compare with analytic solution

src/test-gradient-trivial.cc

 /* Checks if the descent direction is computed correctly using the
    analytic solution; also checks if the algorithm converges
-   regardless of where it starts */
+   to the right solution regardless of where it starts */
 
 #ifdef HAVE_CONFIG_H
 #include "config.h"
@@ -27,6 +27,10 @@ int main() {
   b[0] = 1;
   b[1] = 2;
 
+  Functional::SmallVector solution; // Analytic solution
+  solution[0] = 4.0 / 39.0;
+  solution[1] = 6.0 / 13.0;
+
   auto f = Dune::make_shared<Dune::TrivialFunction const>();
   auto phi = Dune::make_shared<Functional::NonlinearityType const>(f);
   Functional J(A, b, phi);
@@ -49,11 +53,18 @@ int main() {
   assert(error.two_norm() < 1e-10);
 
   double const ret1 = functionTester(J, start, 6);
+  error = solution;
+  error -= start;
+  assert(error.two_norm() < 1e-5);
 
   // Something random
   start[0] = 279;
   start[1] = -96;
 
-  double const ret2 = functionTester(J, start, 16);
+  double const ret2 = functionTester(J, start, 25);
+  error = solution;
+  error -= start;
+  assert(error.two_norm() < 1e-6);
+
   assert(std::abs(ret1 - ret2) < 1e-5);
 }