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# MatCreateSeqSBAIJ
Creates a sparse symmetric matrix in (block compressed row) `MATSEQSBAIJ` format.  For good matrix assembly performance the user should preallocate the matrix storage by setting the parameter `nz` (or the array `nnz`). 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatCreateSeqSBAIJ(MPI_Comm comm, PetscInt bs, PetscInt m, PetscInt n, PetscInt nz, const PetscInt nnz[], Mat *A)
```
Collective


## Input Parameters

- ***comm -*** MPI communicator, set to `PETSC_COMM_SELF`
- ***bs -*** size of block, the blocks are ALWAYS square. One can use `MatSetBlockSizes()` to set a different row and column blocksize but the row
blocksize always defines the size of the blocks. The column blocksize sets the blocksize of the vectors obtained with MatCreateVecs()
- ***m -*** number of rows
- ***n -*** number of columns
- ***nz -*** number of block nonzeros per block row (same for all rows)
- ***nnz -*** array containing the number of block nonzeros in the upper triangular plus
diagonal portion of each block (possibly different for each block row) or `NULL`



## Output Parameter

- ***A -*** the symmetric matrix



## Options Database Keys

- ***-mat_no_unroll -*** uses code that does not unroll the loops in the
block calculations (much slower)
- ***-mat_block_size -*** size of the blocks to use





## Notes
It is recommended that one use the `MatCreate()`, `MatSetType()` and/or `MatSetFromOptions()`,
MatXXXXSetPreallocation() paradigm instead of this routine directly.
[MatXXXXSetPreallocation() is, for example, `MatSeqAIJSetPreallocation()`]

The number of rows and columns must be divisible by blocksize.
This matrix type does not support complex Hermitian operation.

Specify the preallocated storage with either `nz` or `nnz` (not both).
Set `nz` = `PETSC_DEFAULT` and `nnz` = `NULL` for PETSc to control dynamic memory
allocation.  See [Sparse Matrices](sec_matsparse) for details.

If the `nnz` parameter is given then the `nz` parameter is ignored


## See Also
 [](ch_matrices), `Mat`, [Sparse Matrices](sec_matsparse), `MATSEQSBAIJ`, `MatCreate()`, `MatCreateSeqAIJ()`, `MatSetValues()`, `MatCreateSBAIJ()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/sbaij/seq/sbaij.c.html#MatCreateSeqSBAIJ">src/mat/impls/sbaij/seq/sbaij.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/sbaij/seq/sbaij.c)


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