[petsc-users] Algorithms to remove null spaces in a singular system

[Jed Brown]

Fande Kong <fdkong.jd at gmail.com> writes:

> I think we need to make sure that the basis vectors are orthogonal to each
> other and they are normalized. Right?

Yes, that is clearly stated in the man page and checked for in debug
mode.  The relevant code to remove the null space is

  if (sp->n) {
    ierr = VecMDot(vec,sp->n,sp->vecs,sp->alpha);CHKERRQ(ierr);
    for (i=0; i<sp->n; i++) sp->alpha[i] = -sp->alpha[i];
    ierr = VecMAXPY(vec,sp->n,sp->alpha,sp->vecs);CHKERRQ(ierr);
  }
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