presolve leads to a solution to an infeasible problem.

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Iliya Markov

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Mar 7, 2023, 3:57:42 AM3/7/23
to lp_solve
Hello, 

While exploring the lpSolve behavior I stumbled upon a result that I find unexpected. Consider the following problem:

min: 10;
x + y = 0;
x + y >= 300;

The problem is infeasible and lpSolve reports that correctly when I run:

lp_solve input.lp, where input.lp contains the above problem.

However, when I apply

lp_solve input.lp -presolverow -presolvecol

lpSolve reports that the problem has a solution:

Value of objective function: 10.00000000

Actual values of the variables:
x                               0
y                               0

How should I understand the application of the two presolve flags here? Is this a bug?

Any hints appreciated.
Iliya 


Peter Notebaert

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Mar 7, 2023, 6:44:27 AM3/7/23
to Iliya Markov, lp_solve
Unfortunately presolve in lpsolve is experimantial and not very trustworthly.

You can also see the result of presolve by doing this:

lp_solve input.lp -presolverow -presolvecol -wafter -wlp result.lp

Peter


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Iliya Markov

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Mar 7, 2023, 7:31:41 AM3/7/23
to lp_solve
Thanks Peter!
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