[Cbc] SOS2 and presolve

[Stefan Vigerske]

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