[petsc-users] A number of questions about DMDA with SNES and Quasi-Newton methods

[zakaryah .]

OK that is super helpful.  Just to be sure - for MxNxP, the row r in the
Jacobian is at r.i*P*N*3 + r.j*P*3 + r.k*3 + r.c?

On Mon, Sep 4, 2017 at 4:58 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Sep 4, 2017, at 3:48 PM, zakaryah . <zakaryah at gmail.com> wrote:
> >
> > One piece of information that would be useful is what ordering PETSc
> uses for the Jacobian in the snes_test_display.  Is it a natural ordering,
> or the PETSc ordering?  For debugging the Jacobian manually, the natural
> ordering is much easier to work with.
>
>    What is displayed is always the natural ordering (internally it is not
> the natural ordering).
>
> >  For -n 1, are the orderings the same?
>
>   yes
>
>
>
> >
> > If I use a MatStencil r to represent a field with 3 degrees of freedom,
> and the dimensions of my 3D DMDA are MxNxP, which row of the Jacobian
> corresponds to r.i=x, r.j=y, r.k=z, r.c=f?
>
> Internally it is complicated but for any viewing it is just the natural
> ordering and all the degrees of freedom for a single point are next to each
> other in the vector/matrix.
>
>
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