[petsc-users] Shell matrix in petsc4py?

[Gaetan Kenway]

Hi Jonathan

I've successfully used petsc shell matrices from Python. There is an
example of how to do it in demo/poisson2d/poisson2d.py. The example is a
little sparse, but it has all the important information. The most important
is to define the

def mult(self, mat, x, y):

routine inside of a Python class that is given in mat.setPythonContext()
command.

Hope that helps.

Gaetan


On Wed, Jul 30, 2014 at 7:29 PM, Jonathan Tu <jonathantu at gmail.com> wrote:

> Hi,
>
> I would like to solve a large linear system.  I have a linear operator
> that operates on a "state" vector.  The linear operator does things like
> take the sum of the state elements, take the cross product of the state
> with a known vector, and then multiply the state by a large (distributed)
> matrix.  You could imagine it like this:
>
> [sum of elements, cross product with other vector, multiplication with
> large matrix] = A * x
>
> Now, I realize it is possible to implement all of this using a large
> matrix.  After all, these are all linear operations.  However, I thought it
> might be faster and less error-prone to use some of numpy's built-in
> operations, for the sums and cross products, and only use a distributed
> matrix for the last part of the output.
>
> From the PETSc documentation, it seems like a matrix shell would be
> perfect for this.  I could define my own mult operator that does all of
> these things in steps, using built-in numpy functions when appropriate and
> direct matrix multiplication when appropriate.  However, I cannot figure
> out how to do this in petsc4py.  I found somewhere online that perhaps
> createPython is the way I need to go about this, but I couldn't figure out
> the details.
>
> Any ideas?
>
>
>
> Jonathan Tu
>
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