[petsc-users] Integrating SNES in FEM code

[Saransh Saxena]

Hi,

I am trying to incorporate newton method in solving a nonlinear FEM
equation using SNES from PETSc. The overall equation is of the type A(x).x
= b, where b is a vector of external loads, x is the solution field (say
displacements for e.g.) and A is the combined LHS matrix derived from the
discretization of weak formulation of the governing finite element
equation.

While going through the manual and examples of snes, I found that I need to
define the function of residual using SNESSetFunction() and jacobian using
SNESSetJacobian(). In that context I had a couple of questions :-

1. In the snes tutorials I've browsed through, the functions for computing
residual passed had arguments only for x, the solution vector and f, the
residual vector. Is there a way a user can pass an additional vector (b)
and matrix (A) for computing the residual as well? as in my case, f = b -
A(x).x

2. Since computing jacobian is not that trivial, I would like to use one of
the pre-built jacobian methods. Is there any other step other than setting
the 3rd argument in SNESSetJacobian to SNESComputeJacobianDefault?

Best regards,

Saransh
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