[petsc-users] trust region/hook step equivalence

[Dave Lee]

Hi PETSc,

I'm using the SNES trust region to solve a matrix free Newton problem. I
can't see a lot of description of the trust region algorithm in the manual
(section 5.2.2), and have also found it difficult to find documentation on
the MINPACK project from which it is apparently derived. I have a couple of
questions about this:

1) Is the PETSc SNES trust region algorithm the same as the "hook step"
algorithm detailed in Section 6.4.1 of Dennis and Schnabel (1996)
"Numerical methods for Unconstrained Optimization and Nonlinear Equations"?

2) Is there anywhere I can find specific documentation on the trust region
control parameters as defined in:
https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESNEWTONTR.html#SNESNEWTONTR

3) My solve returns before it is sufficiently converged. On the last few
Newton iterations the KSP converges due to:
CONVERGED_STEP_LENGTH
after only a couple of KSP iterations. What is the default for this
parameter?, and how can I change it? Should I change it?

Cheers, Dave.
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