On 20 Apr 2015, at 09:55, Colin Cotter <colin.cotter@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