no variables, but lower bound

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Milan Marinkovic

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Sep 21, 2016, 2:58:54 AM9/21/16
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Hello, I have problem in my work... There is presolve problem with my constrain, and I don't know how to fix it... Please help me...
mancic.dat
mancic.mod

Robert Fourer

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Sep 21, 2016, 3:31:38 PM9/21/16
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The message "Infeasible constraints determined by presolve" tells you that you have an infeasible problem: There is no way to assign values to your variables that will satisfy all of the constraints.

As the "no variables" message suggests, there are no variables at all in the 1b constraints.  In fact 1b['Y1'] reduces to 3 <= 1, which cannot possibly be satisfied; and similarly for 1b['Y2'].

The other constraints mentioned have some variables, but presolve has been able to deduce (from other constraints) fixed values for those variables, after which the constraint is seen to be impossible.  Indeed, if you give AMPL the command "display x.lb, x.ub;" you will see that all of the x-variables have been fixed to zero.  You will need to study the model more closely to determine why 0 is the only possible value for the x-variables, given your constraints; you may find it helpful to use commands like "expand 1c['Y2','V3'];" or "expand 1d;" to see the constraints that AMPL actually generated and compare them to the constraints you intended.

Bob Fourer
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