Obtaining nested Jacobian matrix from nonlinear residual and fieldsplit preconditioning

[Stein]

Maybe this is besides the question that you’re asking, but block preconditioning with fieldsplit is already very doable for mixed spaces, also for nonlinear problems. See for example:

Precondition appears not to happen

Hi all, For a Navier-Stokes problem, I want to separately precondition the u block (Jacobi) and the p block (algebraic multigrid on the Schur complement), while using mixed spaces. I believe I have everything set up correctly, and am (finally ) no longer getting the PETSc warning “There are unused database options”. But when I activate solver verbosity (setting ksp_view), then none of my own settings are mentioned in the output. Instead it mentions LU or MUMPS preconditioners (d…

Other resources that I found quite useful are Rafinex-external-RIFLE / FEniCSx-pctools · GitLab and ambit/src/ambit_fe/solver/preconditioner.py at master · marchirschvogel/ambit · GitHub