[Cbc] SOS2 and presolve

Fri Jun 1 03:22:27 EDT 2007

Stefan,

It is possible, if cumbersome, to tweak preprocessing so that it has a
better chance of leaving SOS.  If you send me the mps file, I can see if I
can fix.

John

             Stefan Vigerske                                               
             <stefan at math.hu-b                                             
             erlin.de>                                                  To 
             Sent by:                  cbc at list.coin-or.org                
             cbc-bounces at list.                                          cc 
             coin-or.org               Michael Bussieck                    
                                       <mbussieck at gams.com>                
                                                                   Subject 
             05/31/07 08:24 AM         [Cbc] SOS2 and presolve             

Hi,

I have been using Cbc on a model that contains some SOS type 2
constraints. In the solution found by Cbc, one of the SOS2 constraints
was violated, that is, two non-succeeding variables in one of the
special ordered sets had a nonzero variable.

I did some debugging and found that some variables had been removed from
the SOS. If I switch off the integer presolve, then the variables are
still there and Cbc finds a solution that satisfies the SOS2 constraints.
So my question is whether this is a bug in the presolve, or it is just
not allowed to use integer presolve in the presence of SOS constraints?

For the integer presolve routine see line 195 at
<
https://projects.coin-or.org/GAMSlinks/browser/trunk/GAMSlinks/src/Cbc/CbcStrategyGams.cpp
>
I copied this from CbcStrategyDefault.

Best,
Stefan

--
Stefan Vigerske
Humboldt University Berlin, Numerical Mathematics
http://www.math.hu-berlin.de/~stefan
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