Block matrices are a special representation of matrices which can save a
lot of memory if large matrices have a block structure with lots of zero
blocks. GAP uses the representation IsBlockMatrixRep to store block
matrices.
AsBlockMatrix( m, nrb, ncb ) F
returns a block matrix with nrb row blocks and ncb column blocks which is equal to the ordinary matrix m.
BlockMatrix( blocks, nrb, ncb ) F
BlockMatrix( blocks, nrb, ncb, rpb, cpb, zero ) F
BlockMatrix returns an immutable matrix in the sparse representation
IsBlockMatrixRep.
The nonzero blocks are described by the list blocks of triples,
the matrix has nrb row blocks and ncb column blocks.
If blocks is empty (i.e., if the matrix is a zero matrix) then the dimensions of the blocks must be entered as rpb and cpb, and the zero element as zero.
Note that all blocks must be ordinary matrices (see IsOrdinaryMatrix), and also the block matrix is an ordinary matrix.
MatrixByBlockMatrix( blockmat ) F
returns an ordinary matrix that is equal to the block matrix blockmat.
GAP 4 manual