[petsc-users] TS question 1: how to stop explicit methods because they do not use SNES(VI)?

[Matthew Knepley]

On Tue, Feb 14, 2017 at 8:55 AM, Emil Constantinescu <emconsta at mcs.anl.gov>
wrote:

> On 2/14/17 6:53 AM, Matthew Knepley wrote:
>
>>
>>     Explicit methods use only RHSFunction and ignore IFunction, so in
>>     your case, if you change TS type to rk and ssp at run time, you are
>>     actually solving u_t = G(t,u). If RHSFunction is not provided, PETSc
>>     will assume the RHS is zero (u_t=0).
>>
>>
>> I do not agree with this. I thought our aim was to support users
>> comparing explicit with implicit with semi-implicit. This breaks that
>> model completely.
>>
>
> Matt, all RHSFunction can be turned into IFunction but not the other way
> around. Explicit TS solvers cannot deal with all forms of IFunction and
> providing support for classifying them may be burdensome and difficult to
> scale. I think it's more lucrative to improve the documentation.


I see that is it not one-to-one, but its a pattern I think a lot of people
will want, and one of the most common
things people ask is "when does implicit beat explicit?". We should think
about ways to make this easier.

   Matt


>
> Emil
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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