Equivalent of LinearVariationProblem and NonlinearVariationalProblem in FEniCSx

Hi everyone,

I am new to the FEniCSx. I am converting one of my code from FEniCS to FEniCSx. I want to know is there any equivalent of LinearVariationProblem and NonlinearVariationalProblem in FEniCSx. I am not able to find these in dolfinx.fem.

Thank you,
Manish

See:
https://jorgensd.github.io/dolfinx-tutorial/chapter1/fundamentals_code.html#forming-and-solving-the-linear-system
and
https://jorgensd.github.io/dolfinx-tutorial/chapter2/nonlinpoisson_code.html#newtons-method

Thank you Prof. Dokken.

I am using PETScTAOSolver to solve the bounded solution problem along with OptimisationProblem class.
is OptimisationProblem class still a part of dolfinx?
in Dolfin: is was class dolfin.cpp.la. OptimisationProblem

Thank you
Manish