[firedrake] initial guess

[Lawrence Mitchell]

> On 20 Apr 2015, at 09:55, Colin Cotter <colin.cotter at imperial.ac.uk> wrote:
> 
> Dear Firedrake list,
>    Does the Firedrake interface to PETSc use the provided solution field as an initial guess by default? By default, a PETSc KSP uses zero as the initial guess.

If you're using a variational solver, since we always solve in residual form:

Ax = b

is transformed into:

F(x) = Ax - b = 0

And any initial non-zero guess is used for the starting position of the newton iteration.

If you're using a LinearSolver object (i.e. preassembled operators).  Pass {'ksp_initial_guess_nonzero': True} to the solver_parameters and the initial guess will be used.  I'm not sure this is tested, so if you have problems please do complain.

Lawrence
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