[petsc-users] Two questions regarding SNESLinesearchPrecheck

[Jed Brown]

I think SNESLineSearchApply_Basic will clarify these points. Note that the pre-check is applied before "taking the step", so X is x_k. You're right that the sign is flipped on search direction Y, as it's using -lambda below.

  /* precheck */
  ierr = SNESLineSearchPreCheck(linesearch,X,Y,&changed_y);CHKERRQ(ierr);

  /* update */
  ierr = VecWAXPY(W,-lambda,Y,X);CHKERRQ(ierr);
  [...]
  ierr = VecCopy(W, X);CHKERRQ(ierr);

David Knezevic <david.knezevic at akselos.com> writes:

> We're using SNESLinesearchPrecheck in order to implement some nonlinear
> continuation methods, and we had a couple of questions:
>
> 1. The "search direction" that is handed to the pre-check function is
> referred to as "y" in the documentation. We had assumed that we would have
> "y = delta_x_k", where delta_x_k is as shown in the attached screenshot
> from the PETSc manual. But after doing some testing it seems that in fact
> we have "y = -delta_x_k", i.e. y is the NEGATIVE of the Newton step. Is
> this correct?
>
> 2. In the documentation for SNESLineSearchPreCheck it says that "x" is the
> "current solution". Referring again to the attached screenshot, does this
> mean that it's x_k+1 or x_k? I assume it means x_k+1 but I wanted to
> confirm this.
>
> Thanks for your help!
>
> Regards,
> David