"Infeasibility Error"

[Arash Khojaste]

This is the error that I'm getting. Does anyone know what I should do?
"CPLEX 20.1.0.0: infeasible problem.
458 dual simplex iterations (1 in phase I)
constraint.dunbdd returned
2628 extra dual simplex iterations for ray

suffix dunbdd OUT;"

[AMPL Google Group]

An "infeasible problem" message from the solver means that there is no way to assign values to the variables that are within the variables' bounds and that also satisfy all of the constraints. This may be due to an error in your model. Or it may be that your data simply doesn't permit a feasible solution; for example, some capacities may be too low to permit all demands to be met.

There is no simple and general way to deal with an error like this; you may need to study the paper and the model for a while before you can understand the cause of the infeasibility. It is possible to suggest some good ways to get started, however.

As an initial troubleshooting step, it is often helpful to use AMPL's expand command to see whether AMPL generated the constraints that you expected. By itself, expand; shows all of the constraints. If there are many of them, you can use, for example, expand C1; to expand all of the constraints that have a particular name. Also you can write, for instance, expand >listing.txt; or expand C1 >listing.txt; to send all of the output to the file listing.txt.

You should also consider using AMPL's drop and restore commands to narrow your search for the cause of the infeasibility. If you drop some constraints and the resulting problem is still infeasible, then you don't need to consider the dropped constraints in looking for the cause of infeasibility. By trying a series of drops, you may be able to determine that infeasibility is being caused by only a small subset of the constraints.

If you are using one of the popular MIP solvers (CPLEX, Gurobi, Xpress) then to get more help with diagnosing the infeasibility, you can ask the solver to compute an irreducible infeasible subset. For more on this topic, search "IIS" at groups.google.com/group/ampl, and also see the discussion of "infeasibility diagnosis" in chapter 14 of the AMPL book (https://ampl.com/BOOK/CHAPTERS/17-solvers.pdf#page=25).

--
Robert Fourer
am...@googlegroups.com

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