solver:implementing_new_branching_rules

Q: * I have implemented a MIP-model in GAMS, which I would like to solve to proven optimality. The problem has a lot of degenerate optimal problems, and solutions very close to the optimum, so it is not terminating with the optimal solution, but continuing to examine nodes with a very small gap > 0.00 %
I have a good idea for a branching rule, which i believe can work around the degeneracy problems. Instead of doing binary branching on fractional binary variables, I intend to branch on the sum of some variables, i.e: \sum_s \delta_s \leq k or \sum_s \delta_s \geq k+1*

*My question is then, how can I make a branching rule as this in GAMS, instead of using binary branching ?? *

Assuming your `\delta_s`

are discrete variables (binary or integer), you could introduce another integer variable `sum_delta =e= sum(s, delta(s));`

Now you can use priorities to tell Cplex to first branch on
the `sum_delta`

variable and then on the `delta(s)`

:

delta.prior(s) = 100; sum_delta.prior = 1; mymodel.prioropt=1; solve mymodel min obj using mip;

If your multiple close optimal solutions come from symmetry try to avoid it. You might try set symmetry cut generation (option ```
symmetry
```

) to very aggressive level (3). Preprocessing at the node or aggressive probing might also help (option `preslvnd`

and `probe`

).

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solver/implementing_new_branching_rules.txt · Last modified: 2013/08/09 08:23 by support