[petsc-dev] question about time integrators, TS

[Hong Zhang]

> On Aug 3, 2016, at 5:32 PM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> So if we are not using IMEX then the semantics of the TS are the same
> if we put the terms (functions and Jacobians) on the LHS (F) or RHS
> (G), with a sign change of course.

Yes.

> And just to be 100% clear, if we put terms in the RHS (function) but
> not in the G Jacobian, then this is like treating these terms with
> forward Euler, is that right?

Yes. If you only want to use an explicit method, you can just provide the RHS function without the RHS Jacobian. 

Hong

> I that right?
> 
> On Wed, Aug 3, 2016 at 3:01 PM, Hong Zhang <hongzhang at anl.gov> wrote:
>> If you use a fully implicit method such as backward Euler, the G Jacobian will be used by PETSc. If you switch to an explicit method, the G Jacobian function will not be used. For IMEX, the G function is treated explicitly, so its Jacobian will not be used.
>> 
>> Hong
>> 
>>> On Aug 3, 2016, at 2:59 PM, Mark Adams <mfadams at lbl.gov> wrote:
>>> 
>>> We have a question about the RHS Jacobian (G) for TS.  The manual says:
>>> 
>>> • Jacobian Gu
>>> If using a fully implicit method and the function G() is provided, one
>>> also can provide an appropriate (approximate) Jacobian matrix of G().
>>> 
>>> If we use, say, backward Euler and provide a G Jacobian, does PETSc
>>> use this G Jacobian in some way? or is the G Jacobian only used for
>>> IMEX methods?
>>> 
>>> Mark
>>