[SCIP] After stopping pricing and doing restart, model is becoming unbounded during presolve?

[Katrin Halbig]

Hi Christian,

It's hard to say where the problem is. Also your logfile looks really 
strange.

If you like, you can send me the lp-file and I will have a closer look 
at this situation. Please also tell me which version of scip you use, so 
that I can reproduce it.

Best regards,
Katrin

Am 20.01.21 um 09:31 schrieb Franzen, Christian:
>
> Hi Ambros.
>
>
> Thanks for your reponse and sorry for answering late. I have "switched 
> off" presolving by including no presolver plugins, hence I only got 
> the presolving code from the propagators and constraint handlers. That 
> seems to work properly. I started to include one presolver plugin 
> after the other and there seems to be a problem with "domcol". It 
> fixed a lot of variables and afterwards my problem is reported to be 
> unbounded. I did not test all presolver plugins yet, but I will try to 
> test all of them by the end of this week. Any idea concerning "domcol"?
>
>
> Regards
> Christian
>
> ------------------------------------------------------------------------
> *Von:* Scip <scip-bounces at zib.de> im Auftrag von Ambros Gleixner 
> <gleixner at zib.de>
> *Gesendet:* Mittwoch, 13. Januar 2021 21:15
> *An:* scip at zib.de
> *Betreff:* Re: [SCIP] After stopping pricing and doing restart, model 
> is becoming unbounded during presolve?
> Hi Christian,
>
> Strange indeed.  Have you tried to turn off presolving?  Does it still
> happen?  Have you tried to successively activate individual presolvers
> to see when it starts failing?
>
> Best,
> Ambros
>
>
> Am 04.01.21 um 17:10 schrieb Franzen, Christian:
> > Hi Scip Team.
> >
> >
> > Thanks to the help of Gregor, I managed to make SCIP stop pricing after
> > some iterations and doing a restart on the reduced master without
> > pricing. Unfortunately, I am facing some really strang behaviour.
> > Sometime it happens, that my model (for that I already have found a
> > feasible solution), is reported to be unbound. I wrote the 
> corresponding
> > LP files before and after presolving to disk and I see that some
> > variable bounds vanished, which seems to be the reason for the
> > unboundness. Furthermore, some of the objective coefficients changed
> > their sign. Any ideas how or why that is happening during presolving?
> >
> >
> > Regards
> >
> > Christian
> >
> >
> > P.S.: I attach the SCIP log for one of these runs.
> >
> >
> > _______________________________________________
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> > Scip at zib.de
> > https://listserv.zib.de/mailman/listinfo/scip
> >
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-- 
Katrin Halbig, FAU Erlangen-Nürnberg, LS Wirtschaftsmathematik, Tel. +49 9131 85-67179

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