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Sometimes a basis may not help and one may find the solver fails or uses excessive iterations. This can be particularly true in a repeated set of solutions with a radically altered model. For example, one can observe messages after a solve such as
sorry guys we seem to be stuck or
after 3 factorizations the basis is singular
or the solver may also not make significant progress.
There are a couple of possible underlying causes of such messages
| • | One may have altered the model size using conditionals which either: |
| — | Eliminated a large number of variables in the basis. |
| — | Deleted a large number of constraints. |
| • | One may also have caused a radical alteration in the model coefficients that compromised the basis. |
| • | One may be using a basis from a model that was radically altered that possesses many features that the model on hand does not have. |
There are several ways of either avoiding the problems such a basis causes or insuring that an advanced basis would be more compatible between related models
| — | Rather than eliminate a variable leave it in the model but alter it's objective function coefficient so it has a very high cost and will not be desirable to include in the solution. |
| — | Rather than eliminate a constraint make it non-binding by for example in the case of =L= constraints adding a very large constant to the right hand side. |
| • | Revert to a saved basis from a related but less altered model. |
| • | Tell the solver to dump the basis using the BRATIO=1 option. |
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