[petsc-users] A matrix-free Jacobian for SNES using MatShell

[Jeremy Roberts]

Hi folks,

I am trying to implement a sample problem for SNES using a shell matrix
construct for the Jacobian (*not* finite differences).

However, I haven't yet found a straightforward example for this.  I've seen
one for a shell-based PC but not for a shell-based Jacobian.

I thought a sequence like this would work, but I'm not sure how to set the
Jacobian.

          call MatCreateShell( PETSC_COMM_SELF, m, n, m, n,
PETSC_NULL_OBJECT, Jshell, ierr )
          call MatShellSetOperation( Jshell, MATOP_MULT, jac_shell, ierr )
          call MatCreateSNESMF( snes, Jshell, ierr )
          call SNESSetJacobian( snes, Jshell, Jshell, ???,
PETSC_NULL_OBJECT, ierr )

I need no PC right now, jac_shell computes the result J*v, and Jshell is an
empty Mat.

How do I finish this calling sequence?  Am I way off base here?

Thanks,
Jeremy
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