<br><font size=2 face="sans-serif">Sathi,</font>
<br>
<br><font size=2 face="sans-serif">Here are some answers.</font>
<br>
<br><tt><font size=2>coin-lpsolver-bounces@list.coin-or.org wrote on 02/15/2007
11:02:30 AM:<br>
<br>
> Hi,<br>
> <br>
> I have an LP problem in which presolve removes a lot of variables.
I have<br>
> attached the presolve statistics for a model I have.<br>
> <br>
> While solving the presolved model, I would like to use some relationships<br>
> between the original variables in guiding the pivoting heuristic.
So I am<br>
> interested in two issues:<br>
> <br>
> 1. Which variables are removed? How do the variables in the presolved<br>
> model map to the original variables?</font></tt>
<br>
<br><tt><font size=2>The ClpPresolve object has methods - originalColumns()and
originalRows() which you can use to find out this information.</font></tt>
<br><tt><font size=2><br>
> <br>
> 2. What values are the removed variables fixed at, if any? Or,
are the<br>
> values of some of the removed variables are only fixed during postsolving<br>
> process?</font></tt>
<br>
<br><tt><font size=2>For some types of variables, the removed variables
will be fixed at their bounds - but there is no easy way to find out which
bound. For presolve operations which get rid of constraints like
x + 5y = 7 then these values are only known during postsolve. In
your case most of the operations are probably the result of constraints
like x + 2y +3z <= 0 in which case they would be fixed to lower bound.</font></tt>
<br><tt><font size=2><br>
> <br>
> I am at present looking at presolve literature and also the COIN presolve<br>
> codes to figure out these issues. Any help would be useful to me.<br>
> <br>
> Thanks,<br>
> Sathi<br>
> <br>
> ___________________________________________________________________<br>
> [attachment "presolve_stats.txt" deleted by John J Forrest/Watson/IBM]
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</font></tt>