[Cbc] bug in CBC or CGL

[John Forrest]

Bug is in presolve - not Cbc or Cgl.

There are 17 0-1 identical columns - presolve makes a single 0-17
column.  Then preprocessing finds this column must be at least 2.
OsiPresolve then says - oh look the lower bound has increased - maybe I
should redo original problem with tighter bounds.  So it puts a lower
bound of 2 on one of the original columns - ouch!

Should be fixed.  Now only goes round again if no merging and new bounds
not integral.

John Forrest




On Thu, 2010-04-01 at 18:56 -0400, Matthew Galati wrote:
> http://coral.ie.lehigh.edu/~magh/tmp/PRICE_AND_CUT_subProb_relax0.n0.c0.p0.b0.lp
> 
> -----------
> Cbc claims the problem is INF or UNB.
> 
> Coin Cbc and Clp Solver version 2.4stable, build Apr  1 2010
> command line - ./COIN/coin-Dip-shark/build-g/bin/cbc /home/magh/PRICE_AND_CUT_subProb_relax0.n0.c0.p0.b0.lp (default strategy 1)
> ### WARNING: CoinLpIO::readLp(): Variable X00096 does not appear in objective function or constraints
> ...
> Continuous objective value is 0 - 0.00 seconds
> Cgl0000I Cut generators found to be infeasible!
> Pre-processing says infeasible or unbounded
> Total time 0.00
> 
> 
> 
> -----------
> Cplex claims the problem is trivially solved.
> 
> CPLEX> opt
> Tried aggregator 1 time.
> MIP Presolve eliminated 5 rows and 101 columns.
> All rows and columns eliminated.
> Presolve time =    0.00 sec.
> 
> Solution pool: 1 solution saved.
> 
> MIP - Integer optimal solution:  Objective =  0.0000000000e+00
> Solution time =    0.00 sec.  Iterations = 0  Nodes = 0
> 
> 
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