# Re: [petsc-users] Example for MatInvertBlockDiagonal

```> On Sep 19, 2016, at 10:43 AM, Cyrill Vonplanta <cyrill.von.pla...@usi.ch>
> wrote:
>
> Barry,
>
> Thanks a lot. I’d like to use this for a nonlinear variant of a
> block-gauss-seidel smoother. I would like to use MatInvertBlockDiagonal for
> speeding up my variant.
>
> I think I can work with this, however I also have the problem to turn my
> initial matrix into one with a blocksize of 3.When I call:
>
> MatConvert(A, MATBAIJ, MAT_INITIAL_MATRIX, Dinverse);```
```
MatConvert() creates a new matrix so you should not create the Dinverse
beforehand; anything you put in Dinverse before is lost..m

block size > 1 really only makes sense if the block size is really greater
than one. So if A has blocks of size 3 you should create A as BAIJ and thus
never need to call the convert routine.

You can also set the block size for AIJ matrix to 3 and use
MatInvertBlockDiagonal() on that matrix and not use the BAIJ matrix.

Finally note that MatInvertBlockDiagonal() ends up calling (for block size
3) PetscKernel_A_gets_inverse_A_3() for each block. It sounds to me like that
is what you would want for nonlinear variant of a block-gauss-seidel smoother..

Barry

>
>
> Then the matrix Dinverse has blocksize 1 which comes from A. I checked the
> blocksize before the conversion and it was 3, so it seems to get lost.
>
> What is the correct (and elegant) way to turn a matrix into a block matrix?
>
>
> Best
> Cyrill
>
>
>
> On 19/09/16 17:18, "Barry Smith" <bsm...@mcs.anl.gov> wrote:
>
>>
>> Cyrill,
>>
>>   This is very specialized for implementing point block Jacobi; I don't
>> think it is something you would want to use directly.
>>
>>    If you do want to use it, it simply returns the inverses of the block
>> diagonals in column major form. You can then call MatSetValues() with with
>> each of those blocks into another PETSc matrix. values[i*bs*bs] is the
>> starting point of each block in the array.
>>
>>  Barry
>>
>>> On Sep 19, 2016, at 4:55 AM, Cyrill Vonplanta <cyrill.von.pla...@usi.ch>
>>> wrote:
>>>
>>> Dear PETSc-Users,
>>>
>>> I would like to use the inverted block diagonals of a a matrix. I have seen
>>> the function MatInvertBlockDiagonal() but I don’t know how to create a
>>> matrix out of them or an array of block matrizes.
>>>
>>> Does anyone have an example on how to use  **values to create a PETSc
>>> matrix?
>>>
>>> Thanks
>>> Cyrill
>>

```