[petsc-users] Problems with the null-space removal

[MIGUEL MOLINOS PEREZ]

Dear all,

I am working on the resolution of a non-linear problem via SNES with a large null-space (at least 70% of the equations are trivial). In a nutshell I have a mesh with active and non-active nodes, and the non-active nodes introduces the null-space. 

So far I've dealing with it using NGMRES but I want to use a scheme based on NR. However, despite removing the null-space using PETSc's in-build functions the linear solver (KSP+PC) diverges at the first iteration. However, If I remove the non-active nodes and use the same solver (SNES+KSP+PC), which failed before, now I can solve the non-linear system.

Any clue? I’m missing something?

Thanks,
Miguel