Add matrix-free solver example

unsupported/Eigen/src/IterativeSolvers/DGMRES.h

@@ -83,6 +83,8 @@ void sortWithPermutation (VectorType& vec, IndexType& perm, typename IndexType::
  * x = solver.solve(b);
  * \endcode
  * 
+ * DGMRES can also be used in a matrix-free context, see the following \link MatrixfreeSolverExample example \endlink.
+ *
  * References :
  * [1] D. NUENTSA WAKAM and F. PACULL, Memory Efficient Hybrid
  *  Algebraic Solvers for Linear Systems Arising from Compressible

unsupported/Eigen/src/IterativeSolvers/GMRES.h

@@ -251,6 +251,8 @@ struct traits<GMRES<_MatrixType,_Preconditioner> >
   * By default the iterations start with x=0 as an initial guess of the solution.
   * One can control the start using the solveWithGuess() method.
   * 
+  * GMRES can also be used in a matrix-free context, see the following \link MatrixfreeSolverExample example \endlink.
+  *
   * \sa class SimplicialCholesky, DiagonalPreconditioner, IdentityPreconditioner
   */
 template< typename _MatrixType, typename _Preconditioner>

unsupported/Eigen/src/IterativeSolvers/MINRES.h

@@ -191,6 +191,8 @@ namespace Eigen {
      * By default the iterations start with x=0 as an initial guess of the solution.
      * One can control the start using the solveWithGuess() method.
      *
+     * MINRES can also be used in a matrix-free context, see the following \link MatrixfreeSolverExample example \endlink.
+     *
      * \sa class ConjugateGradient, BiCGSTAB, SimplicialCholesky, DiagonalPreconditioner, IdentityPreconditioner
      */
     template< typename _MatrixType, int _UpLo, typename _Preconditioner>