#include <multi_poisson_block_preconditioners.h>

Inheritance diagram for oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >:

Public Member Functions
	TwoPlusThreeUpperTriangularWithReplace ()
	Constructor for TwoPlusThreeUpperTriangularWithReplace. More...

	~TwoPlusThreeUpperTriangularWithReplace ()
	Destructor clean up memory. More...

virtual void	clean_up_my_memory ()
	Clean up the memory. More...

	TwoPlusThreeUpperTriangularWithReplace (const TwoPlusThreeUpperTriangularWithReplace &)
	Broken copy constructor. More...

void	operator= (const TwoPlusThreeUpperTriangularWithReplace &)
	Broken assignment operator. More...

void	preconditioner_solve (const DoubleVector &r, DoubleVector &z)
	Apply preconditioner to r, i.e. return z such that P z = r. More...

void	setup ()
	Setup the preconditioner. More...

void	set_multi_poisson_mesh (Mesh *multi_poisson_mesh_pt)
	Specify the mesh that contains multi-poisson elements. More...

Public Member Functions inherited from oomph::BlockPreconditioner< MATRIX >
	BlockPreconditioner ()
	Constructor. More...

virtual	~BlockPreconditioner ()
	Destructor. More...

	BlockPreconditioner (const BlockPreconditioner &)=delete
	Broken copy constructor. More...

void	operator= (const BlockPreconditioner &)=delete
	Broken assignment operator. More...

MATRIX *	matrix_pt () const

void	turn_on_recursive_debug_flag ()

void	turn_off_recursive_debug_flag ()

void	turn_on_debug_flag ()
	Toggles on the debug flag. More...

void	turn_off_debug_flag ()
	Toggles off the debug flag. More...

void	turn_into_subsidiary_block_preconditioner (BlockPreconditioner< MATRIX > *master_block_prec_pt, const Vector< unsigned > &doftype_in_master_preconditioner_coarse)

void	turn_into_subsidiary_block_preconditioner (BlockPreconditioner< MATRIX > *master_block_prec_pt, const Vector< unsigned > &doftype_in_master_preconditioner_coarse, const Vector< Vector< unsigned >> &doftype_coarsen_map_coarse)

virtual void	block_setup ()

void	block_setup (const Vector< unsigned > &dof_to_block_map)

void	get_block (const unsigned &i, const unsigned &j, MATRIX &output_matrix, const bool &ignore_replacement_block=false) const

MATRIX	get_block (const unsigned &i, const unsigned &j, const bool &ignore_replacement_block=false) const

void	set_master_matrix_pt (MATRIX *in_matrix_pt)
	Set the matrix_pt in the upper-most master preconditioner. More...

void	get_block_other_matrix (const unsigned &i, const unsigned &j, MATRIX *in_matrix_pt, MATRIX &output_matrix)

void	get_blocks (DenseMatrix< bool > &required_blocks, DenseMatrix< MATRIX * > &block_matrix_pt) const

void	get_dof_level_block (const unsigned &i, const unsigned &j, MATRIX &output_block, const bool &ignore_replacement_block=false) const

MATRIX	get_concatenated_block (const VectorMatrix< BlockSelector > &selected_block)

void	get_concatenated_block_vector (const Vector< unsigned > &block_vec_number, const DoubleVector &v, DoubleVector &b)

void	return_concatenated_block_vector (const Vector< unsigned > &block_vec_number, const DoubleVector &b, DoubleVector &v) const
	Takes concatenated block ordered vector, b, and copies its. More...

void	get_block_vectors (const Vector< unsigned > &block_vec_number, const DoubleVector &v, Vector< DoubleVector > &s) const

void	get_block_vectors (const DoubleVector &v, Vector< DoubleVector > &s) const

void	return_block_vectors (const Vector< unsigned > &block_vec_number, const Vector< DoubleVector > &s, DoubleVector &v) const

void	return_block_vectors (const Vector< DoubleVector > &s, DoubleVector &v) const

void	get_block_vector (const unsigned &n, const DoubleVector &v, DoubleVector &b) const

void	return_block_vector (const unsigned &n, const DoubleVector &b, DoubleVector &v) const

void	get_block_ordered_preconditioner_vector (const DoubleVector &v, DoubleVector &w)

void	return_block_ordered_preconditioner_vector (const DoubleVector &w, DoubleVector &v) const

unsigned	nblock_types () const
	Return the number of block types. More...

unsigned	ndof_types () const
	Return the total number of DOF types. More...

const Mesh *	mesh_pt (const unsigned &i) const

unsigned	nmesh () const

int	block_number (const unsigned &i_dof) const
	Return the block number corresponding to a global index i_dof. More...

int	index_in_block (const unsigned &i_dof) const

const LinearAlgebraDistribution *	block_distribution_pt (const unsigned &b) const
	Access function to the block distributions (const version). More...

LinearAlgebraDistribution *	block_distribution_pt (const unsigned b)
	Access function to the block distributions (non-const version). More...

LinearAlgebraDistribution *	dof_block_distribution_pt (const unsigned &b)
	Access function to the dof-level block distributions. More...

const LinearAlgebraDistribution *	master_distribution_pt () const

unsigned	ndof_types_in_mesh (const unsigned &i) const

bool	is_subsidiary_block_preconditioner () const

bool	is_master_block_preconditioner () const

void	set_block_output_to_files (const std::string &basefilename)

void	disable_block_output_to_files ()
	Turn off output of blocks (by clearing the basefilename string). More...

bool	block_output_on () const
	Test if output of blocks is on or not. More...

void	output_blocks_to_files (const std::string &basefilename, const unsigned &precision=8) const

void	post_block_matrix_assembly_partial_clear ()

BlockPreconditioner< MATRIX > *	master_block_preconditioner_pt () const
	Access function to the master block preconditioner pt. More...

void	clear_block_preconditioner_base ()

void	document ()

Vector< Vector< unsigned > >	doftype_coarsen_map_fine () const

Vector< unsigned >	get_fine_grain_dof_types_in (const unsigned &i) const

unsigned	nfine_grain_dof_types_in (const unsigned &i) const

MapMatrix< unsigned, CRDoubleMatrix * >	replacement_dof_block_pt () const
	Access function to the replaced dof-level blocks. More...

void	setup_matrix_vector_product (MatrixVectorProduct matvec_prod_pt, CRDoubleMatrix block_pt, const Vector< unsigned > &block_col_indices)

void	setup_matrix_vector_product (MatrixVectorProduct matvec_prod_pt, CRDoubleMatrix block_pt, const unsigned &block_col_index)

void	internal_get_block_ordered_preconditioner_vector (const DoubleVector &v, DoubleVector &w) const

void	internal_return_block_ordered_preconditioner_vector (const DoubleVector &w, DoubleVector &v) const

unsigned	internal_nblock_types () const

unsigned	internal_ndof_types () const

void	internal_return_block_vector (const unsigned &n, const DoubleVector &b, DoubleVector &v) const

void	internal_get_block_vector (const unsigned &n, const DoubleVector &v, DoubleVector &b) const

void	internal_get_block_vectors (const Vector< unsigned > &block_vec_number, const DoubleVector &v, Vector< DoubleVector > &s) const

void	internal_get_block_vectors (const DoubleVector &v, Vector< DoubleVector > &s) const

void	internal_return_block_vectors (const Vector< unsigned > &block_vec_number, const Vector< DoubleVector > &s, DoubleVector &v) const

void	internal_return_block_vectors (const Vector< DoubleVector > &s, DoubleVector &v) const

void	internal_get_block (const unsigned &i, const unsigned &j, MATRIX &output_block) const

int	internal_block_number (const unsigned &i_dof) const

int	internal_index_in_block (const unsigned &i_dof) const

const LinearAlgebraDistribution *	internal_block_distribution_pt (const unsigned &b) const
	Access function to the internal block distributions. More...

void	insert_auxiliary_block_distribution (const Vector< unsigned > &block_vec_number, LinearAlgebraDistribution *dist_pt)

void	block_matrix_test (const unsigned &i, const unsigned &j, const MATRIX *block_matrix_pt) const

template<typename myType >
int	get_index_of_value (const Vector< myType > &vec, const myType val, const bool sorted=false) const

void	internal_get_block (const unsigned &block_i, const unsigned &block_j, CRDoubleMatrix &output_block) const

void	get_dof_level_block (const unsigned &block_i, const unsigned &block_j, CRDoubleMatrix &output_block, const bool &ignore_replacement_block) const

Public Member Functions inherited from oomph::Preconditioner
	Preconditioner ()
	Constructor. More...

	Preconditioner (const Preconditioner &)=delete
	Broken copy constructor. More...

void	operator= (const Preconditioner &)=delete
	Broken assignment operator. More...

virtual	~Preconditioner ()
	Destructor (empty) More...

virtual void	preconditioner_solve_transpose (const DoubleVector &r, DoubleVector &z)

void	setup (DoubleMatrixBase *matrix_pt)

void	setup (const Problem problem_pt, DoubleMatrixBase matrix_pt)

void	enable_silent_preconditioner_setup ()
	Set up the block preconditioner quietly! More...

void	disable_silent_preconditioner_setup ()
	Be verbose in the block preconditioner setup. More...

virtual void	clean_up_memory ()
	Clean up memory (empty). Generic interface function. More...

virtual void	set_matrix_pt (DoubleMatrixBase *matrix_pt)
	Set the matrix pointer. More...

virtual const OomphCommunicator *	comm_pt () const
	Get function for comm pointer. More...

virtual void	set_comm_pt (const OomphCommunicator *const comm_pt)
	Set the communicator pointer. More...

double	setup_time () const
	Returns the time to setup the preconditioner. More...

virtual void	turn_into_subsidiary_block_preconditioner (BlockPreconditioner< CRDoubleMatrix > *master_block_prec_pt, const Vector< unsigned > &doftype_in_master_preconditioner_coarse)

virtual void	turn_into_subsidiary_block_preconditioner (BlockPreconditioner< CRDoubleMatrix > *master_block_prec_pt, const Vector< unsigned > &doftype_in_master_preconditioner_coarse, const Vector< Vector< unsigned >> &doftype_coarsen_map_coarse)

Public Member Functions inherited from oomph::DistributableLinearAlgebraObject
	DistributableLinearAlgebraObject ()
	Default constructor - create a distribution. More...

	DistributableLinearAlgebraObject (const DistributableLinearAlgebraObject &matrix)=delete
	Broken copy constructor. More...

void	operator= (const DistributableLinearAlgebraObject &)=delete
	Broken assignment operator. More...

virtual	~DistributableLinearAlgebraObject ()
	Destructor. More...

LinearAlgebraDistribution *	distribution_pt () const
	access to the LinearAlgebraDistribution More...

unsigned	nrow () const
	access function to the number of global rows. More...

unsigned	nrow_local () const
	access function for the num of local rows on this processor. More...

unsigned	nrow_local (const unsigned &p) const
	access function for the num of local rows on this processor. More...

unsigned	first_row () const
	access function for the first row on this processor More...

unsigned	first_row (const unsigned &p) const
	access function for the first row on this processor More...

bool	distributed () const
	distribution is serial or distributed More...

bool	distribution_built () const

void	build_distribution (const LinearAlgebraDistribution *const dist_pt)

void	build_distribution (const LinearAlgebraDistribution &dist)

Private Attributes
Preconditioner *	First_subsidiary_preconditioner_pt

Preconditioner *	Second_subsidiary_preconditioner_pt

MatrixVectorProduct *	Off_diagonal_matrix_vector_product_pt

DenseMatrix< CRDoubleMatrix * >	Replacement_matrix_pt

Mesh *	Multi_poisson_mesh_pt

Additional Inherited Members
Protected Member Functions inherited from oomph::BlockPreconditioner< MATRIX >
void	set_nmesh (const unsigned &n)

void	set_mesh (const unsigned &i, const Mesh *const mesh_pt, const bool &allow_multiple_element_type_in_mesh=false)

void	set_replacement_dof_block (const unsigned &block_i, const unsigned &block_j, CRDoubleMatrix *replacement_dof_block_pt)

bool	any_mesh_distributed () const

int	internal_dof_number (const unsigned &i_dof) const

unsigned	internal_index_in_dof (const unsigned &i_dof) const

unsigned	internal_block_dimension (const unsigned &b) const

unsigned	internal_dof_block_dimension (const unsigned &i) const

unsigned	master_nrow () const

unsigned	internal_master_dof_number (const unsigned &b) const

const LinearAlgebraDistribution *	internal_preconditioner_matrix_distribution_pt () const

const LinearAlgebraDistribution *	preconditioner_matrix_distribution_pt () const

Protected Member Functions inherited from oomph::DistributableLinearAlgebraObject
void	clear_distribution ()

Protected Attributes inherited from oomph::BlockPreconditioner< MATRIX >
MapMatrix< unsigned, CRDoubleMatrix * >	Replacement_dof_block_pt
	The replacement dof-level blocks. More...

Vector< LinearAlgebraDistribution * >	Block_distribution_pt
	The distribution for the blocks. More...

Vector< Vector< unsigned > >	Block_to_dof_map_coarse

Vector< Vector< unsigned > >	Block_to_dof_map_fine
	Mapping for the block types to the most fine grain dof types. More...

Vector< Vector< unsigned > >	Doftype_coarsen_map_coarse

Vector< Vector< unsigned > >	Doftype_coarsen_map_fine

Vector< LinearAlgebraDistribution * >	Internal_block_distribution_pt
	Storage for the default distribution for each internal block. More...

Vector< LinearAlgebraDistribution * >	Dof_block_distribution_pt

Vector< unsigned >	Allow_multiple_element_type_in_mesh

Vector< const Mesh * >	Mesh_pt

Vector< unsigned >	Ndof_types_in_mesh

unsigned	Internal_nblock_types

unsigned	Internal_ndof_types

Protected Attributes inherited from oomph::Preconditioner
bool	Silent_preconditioner_setup
	Boolean to indicate whether or not the build should be done silently. More...

std::ostream *	Stream_pt
	Pointer to the output stream – defaults to std::cout. More...

Detailed Description

template<typename MATRIX>
class oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >

///////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////// ///////////////////////////////////////////////////////////////////////////// Block diagonal preconditioner for system with 5 dof types assembled into a 2x2 block system, with (0,0) block containing the first two dof types, the (1,1) block the remaining dof types. The blocks are solved by upper block triangular preconditioners. However, the overall system is modified by replacing all off-diagonal blocks by replacement matrices (zero matrices, so the preconditioner again behaves like a 5x5 block diagonal preconditioner).

Constructor & Destructor Documentation

◆ TwoPlusThreeUpperTriangularWithReplace() [1/2]

template<typename MATRIX >

oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::TwoPlusThreeUpperTriangularWithReplace ( )

inline

Constructor for TwoPlusThreeUpperTriangularWithReplace.

                                            : 
    BlockPreconditioner<MATRIX>(),
    First_subsidiary_preconditioner_pt(0),
    Second_subsidiary_preconditioner_pt(0),
    Off_diagonal_matrix_vector_product_pt(0)
    {    
     Multi_poisson_mesh_pt=0;
    } // end_of_constructor

References oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::Multi_poisson_mesh_pt.

◆ ~TwoPlusThreeUpperTriangularWithReplace()

template<typename MATRIX >

oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::~TwoPlusThreeUpperTriangularWithReplace ( )

inline

Destructor clean up memory.

    {
     this->clean_up_my_memory();
    }

References oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::clean_up_my_memory().

◆ TwoPlusThreeUpperTriangularWithReplace() [2/2]

template<typename MATRIX >

oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::TwoPlusThreeUpperTriangularWithReplace ( const TwoPlusThreeUpperTriangularWithReplace< MATRIX > & )

inline

Broken copy constructor.

    { 
     BrokenCopy::
      broken_copy("TwoPlusThreeUpperTriangularWithReplace");
    } 

References oomph::BrokenCopy::broken_copy().

Member Function Documentation

◆ clean_up_my_memory()

template<typename MATRIX >

void oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::clean_up_my_memory

virtual

Clean up the memory.

The clean up function.

  {     
   // Clean up subsidiary preconditioners.
   if(First_subsidiary_preconditioner_pt!=0)
    {
     delete First_subsidiary_preconditioner_pt;
     First_subsidiary_preconditioner_pt = 0;
    }
   if(Second_subsidiary_preconditioner_pt!=0)
    {
     delete Second_subsidiary_preconditioner_pt;
     Second_subsidiary_preconditioner_pt = 0;
    }
  
   // Clean up the replacement matricies.
   for(unsigned i=0,ni=Replacement_matrix_pt.nrow();i<ni;i++)
    {
     for(unsigned j=0,nj=Replacement_matrix_pt.ncol();j<nj;j++)
      {
       if(Replacement_matrix_pt(i,j)!=0)
        {
         delete Replacement_matrix_pt(i,j);
         Replacement_matrix_pt(i,j)=0;
        }
      } // End loop over j.
    } // End loop over i.
     
   // Clean up the off diag matrix products.
   if(Off_diagonal_matrix_vector_product_pt != 0)
    {
     delete Off_diagonal_matrix_vector_product_pt;
     Off_diagonal_matrix_vector_product_pt = 0;
    }
  } // End of clean_up_my_memory function.

References i, and j.

Referenced by oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::~TwoPlusThreeUpperTriangularWithReplace().

◆ operator=()

template<typename MATRIX >

void oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::operator= ( const TwoPlusThreeUpperTriangularWithReplace< MATRIX > & )

inline

Broken assignment operator.

    {
     BrokenCopy::
      broken_assign("TwoPlusThreeUpperTriangularWithReplace");
    }

References oomph::BrokenCopy::broken_assign().

◆ preconditioner_solve()

template<typename MATRIX >

void oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::preconditioner_solve	(	const DoubleVector &	r,
		DoubleVector &	z
	)

virtual

Apply preconditioner to r, i.e. return z such that P z = r.

Preconditioner solve for the two plus three upper triangular with replacement preconditioner: Apply preconditioner to r and return z, so that P r = z, where P is the block diagonal matrix constructed from the original linear system.

Implements oomph::Preconditioner.

  {   
   // Now apply the subsidiary block preconditioner that acts on the
   // "bottom right" 3x3 sub-system (only!). The subsidiary preconditioner 
   // will extract the relevant (3x1) "sub-vectors" from the "big" (5x1)
   // vector big_r and treat it as the rhs, r, of P z = r
   // where P is 3x3 block diagonal.
   Second_subsidiary_preconditioner_pt->preconditioner_solve(r,z);
    
   // Split up rhs vector into sub-vectors, re-arranged to match
   // the matrix blocks
   DoubleVector r_0;
   this->get_block_vector(0,r,r_0);
      
   DoubleVector z_1;
   this->get_block_vector(1,z,z_1);
  
   // Multiply by (0,1) off diagonal.
   DoubleVector temp;
   Off_diagonal_matrix_vector_product_pt->multiply(z_1,temp);
   r_0 -= temp;
  
   // Block solve for first diagonal block. Since the associated subsidiary 
   // preconditioner is a block preconditioner itself, it will extract 
   // the required (2x1) block rhs from a "big" (5x1) rhs vector, big_r.
   // Therefore we first put the actual (2x1) rhs vector block_r_0 into the
   // "big" (5x1) vector big_r. 
   DoubleVector big_r(z.distribution_pt());
   this->return_block_vector(0,r_0,big_r);
     
   // Now apply the subsidiary block preconditioner that acts on the
   // "top left" 2x2 sub-system (only!). The subsidiary preconditioner 
   // will extract the relevant (2x1) "sub-vectors" from the "big" (5x1)
   // vector big_r and treat it as the rhs, r, of P z = r
   // where P is 2x2 block diagonal. Once the system is solved,
   // the result is automatically put back into the appropriate places 
   // of the "big" (5x1) vector z:
   First_subsidiary_preconditioner_pt->preconditioner_solve(big_r,z);
   
  }

References oomph::DistributableLinearAlgebraObject::distribution_pt(), and UniformPSDSelfTest::r.

◆ set_multi_poisson_mesh()

template<typename MATRIX >

void oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::set_multi_poisson_mesh ( Mesh * multi_poisson_mesh_pt )

inline

Specify the mesh that contains multi-poisson elements.

   {
    Multi_poisson_mesh_pt=multi_poisson_mesh_pt;
   }

References oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::Multi_poisson_mesh_pt.

Referenced by MultiPoissonProblem< ELEMENT >::MultiPoissonProblem().

◆ setup()

template<typename MATRIX >

void oomph::TwoPlusThreeUpperTriangularWithReplace< MATRIX >::setup

virtual

Setup the preconditioner.

The setup function.

Implements oomph::Preconditioner.

  {
   // Clean up memory.
   this->clean_up_my_memory();
  
 #ifdef PARANOID
   if (Multi_poisson_mesh_pt == 0)
    {
     std::stringstream err;
     err << "Please set pointer to mesh using set_multi_poisson_mesh(...).\n";
     throw OomphLibError(err.str(),
                         OOMPH_CURRENT_FUNCTION,
                         OOMPH_EXCEPTION_LOCATION);
    }
 #endif 
   
   // The preconditioner works with one mesh; set it!
   this->set_nmesh(1);
   this->set_mesh(0,Multi_poisson_mesh_pt);
  
   // How many dof types do we have?
   const unsigned n_dof_types = this->ndof_types();
  
 #ifdef PARANOID
   // This preconditioner only works for 5 dof types
   if (n_dof_types!=5)
    {
     std::stringstream tmp;
     tmp << "This preconditioner only works for problems with 5 dof types\n"
         << "Yours has " << n_dof_types;
     throw OomphLibError(tmp.str(),
                         OOMPH_CURRENT_FUNCTION,
                         OOMPH_EXCEPTION_LOCATION);
    }
 #endif
    
   // Call block setup with the Vector [0,0,1,1,1] to:
   // Merge DOF types 0 and 1 into block type 0
   // Merge DOF types 2, 3, and 4 into block type 1.
   Vector<unsigned> dof_to_block_map(n_dof_types,0);
   dof_to_block_map[0] = 0;
   dof_to_block_map[1] = 0;
   dof_to_block_map[2] = 1;
   dof_to_block_map[3] = 1;
   dof_to_block_map[4] = 1;
   this->block_setup(dof_to_block_map);
  
 #ifdef PARANOID
  
   // We should now have two block types -- do we?
   const unsigned nblocks = this->nblock_types();
   if (nblocks!=2)
    {
     std::stringstream tmp;
     tmp << "Expected number of block types is 2.\n"
         << "Yours has " << nblocks << ".\n"
         << "Perhaps your argument to block_setup(...) is not correct.\n";
     throw OomphLibError(tmp.str(),
                         OOMPH_CURRENT_FUNCTION,
                         OOMPH_EXCEPTION_LOCATION);
    }
  
 #endif
  
   // Now replace all the off-diagonal DOF blocks.
  
   // Storage for the replacement DOF blocks
   Replacement_matrix_pt.resize(n_dof_types,n_dof_types,0);
  
   // Set off-diagonal DOF blocks to zero, loop over the number of DOF blocks.
   // NOTE: There are two (compound) blocks, but the replacement functionality
   // works with DOF blocks.
   for(unsigned i=0;i<n_dof_types;i++)
    {
     for(unsigned j=0;j<n_dof_types;j++)
      {
       if(i!=j)
        {
         // Modify matrix
         bool modify_existing_matrix=true;
         if (modify_existing_matrix)
          {
           // Get the dof-block and make a deep copy of it
           Replacement_matrix_pt(i,j)=new CRDoubleMatrix;
           this->get_dof_level_block(i,j,(*Replacement_matrix_pt(i,j))); 
           
           // Set all its entries to zero
           unsigned nnz=Replacement_matrix_pt(i,j)->nnz();
           for (unsigned k=0;k<nnz;k++)
            {
             Replacement_matrix_pt(i,j)->value()[k]=0.0;
            }
          } // done -- quite wasteful, we're actually storing lots of zeroes, but
            // this is just an example!
  
         // Build (zero) replacement matrix from scratch:
         else
          {
           // Get the DOF block's (row!) distribution.
           LinearAlgebraDistribution* dof_block_dist_pt=
            this->dof_block_distribution_pt(i);
           
           // Number of rows in DOF block matrix (i,j).
           const unsigned long dof_block_nrow_local 
            = dof_block_dist_pt->nrow_local();
           
           // Number of columns in DOF block matrix (i,j) is the same as the
           // number of rows in block matrix (j,i).
           const unsigned long dof_block_ncol 
            = this->dof_block_distribution_pt(j)->nrow();
           
           // Storage for replacement matrices:
           // Values
           Vector<double> replacement_value(0);
           // Column index
           Vector<int> replacement_column_index(0);
           // Row start
           Vector<int> replacement_row_start;
           
           // Need one row start per row, and one for the nnz, all of which are 0.
           // There are no rows, so this is a Vector of size 1.
           replacement_row_start.resize(dof_block_nrow_local+1,0);
           
           Replacement_matrix_pt(i,j)=
            new CRDoubleMatrix(dof_block_dist_pt, 
                               dof_block_ncol, 
                               replacement_value,
                               replacement_column_index, 
                               replacement_row_start);
          }
         
         // Replace (i,j)-th dof block
         this->set_replacement_dof_block(i,j,Replacement_matrix_pt(i,j));
        }
      }// end for loop of j
    }// end for loop of i
   
   
   // First subsidiary precond is a block triangular preconditioner
   {
    UpperTriangular<CRDoubleMatrix>* block_prec_pt=
     new UpperTriangular<CRDoubleMatrix>;
    First_subsidiary_preconditioner_pt=block_prec_pt;
   
    // Set mesh
    block_prec_pt->set_multi_poisson_mesh(Multi_poisson_mesh_pt);
    
    // Turn it into a subsidiary preconditioner, declaring which
    // of the five dof types in the present (master) preconditioner
    // correspond to the dof types in the subsidiary block preconditioner
    unsigned n_sub_dof_types=2;
    Vector<unsigned> dof_map(n_sub_dof_types);
    dof_map[0]=0;
    dof_map[1]=1;
    block_prec_pt->turn_into_subsidiary_block_preconditioner(this,dof_map);
  
     // Perform setup. Note that because the subsidiary
    // preconditioner is a block preconditioner itself it is given
    // the pointer to the "full" matrix
    block_prec_pt->setup(this->matrix_pt());
   }
  
   // Second subsidiary precond is a block triangular preconditioner
   {
    UpperTriangular<CRDoubleMatrix>* block_prec_pt=
     new UpperTriangular<CRDoubleMatrix>;
    Second_subsidiary_preconditioner_pt=block_prec_pt;
    
    // Set mesh
    block_prec_pt->set_multi_poisson_mesh(Multi_poisson_mesh_pt);
    
    // Turn it into a subsidiary preconditioner, declaring which
    // of the five dof types in the present (master) preconditioner
    // correspond to the dof types in the subsidiary block preconditioner
    unsigned n_sub_dof_types=3;
    Vector<unsigned> dof_map(n_sub_dof_types);
    dof_map[0]=2;
    dof_map[1]=3;
    dof_map[2]=4;
    block_prec_pt->turn_into_subsidiary_block_preconditioner(this,dof_map);
    
    // Perform setup. Note that because the subsidiary
    // preconditioner is a block preconditioner itself it is given
    // the pointer to the "full" matrix
    block_prec_pt->setup(this->matrix_pt());
   }
  
   // Next setup the off diagonal mat vec operators:
   {
    // Get the block
    CRDoubleMatrix block_matrix = this->get_block(0,1);
    
    // Create matrix vector product operator
    Off_diagonal_matrix_vector_product_pt = new MatrixVectorProduct;
    
    // Setup: Final argument indicates block column in the present
    // block preconditioner (which views the system matrix as comprising
    // 2x2 blocks).
    unsigned block_column_index=1;
    this->setup_matrix_vector_product(
    Off_diagonal_matrix_vector_product_pt,&block_matrix,block_column_index);
  
    // Extracted block can now go out of scope since the matrix vector
    // product retains whatever information it needs
   }
  
  }