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

[Barry Smith]

> 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.