[petsc-users] Problems about SNES

[Yingjie Wu]

Dear PETSc developers:
Hi,
During the process of testing the program, I found some questions about
SNES. These are some basic questions that I have overlooked. Please help me
to answer them.
1. Because my program uses - snes_mf_operator, there is no Jacobian matrix.
Linear and non-linear step residuals are different in petsc. The linear
step residuals are r_linear = J*δx-f(x). Since I don't have a Jacobian
matrix, I don't know how to calculate the relative residuals of linear
steps provided in petsc. Do we use the finite difference approximation
matrix vector product when calculating the residuals?
2. Read the user's manual for a brief introduction to the inexact Newton
method, but I am very interested in the use of this method. I want to know
how to use this method in petsc.
3. The default line search used by SNES in PETSc is bt, which often fails
in program debugging. I don't know much about linesearch, and I'm curious
to know why it failed. How can I supplement this knowledge?

Thanks,
Yingjie
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