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## Re: [Help-glpk] Identical variables

**From**: |
Andrew Makhorin |

**Subject**: |
Re: [Help-glpk] Identical variables |

**Date**: |
Fri, 19 Dec 2008 10:20:01 +0300 |

>* However, it doesn't seem like the pre-processing removes the additional*
>* variables, which I sort of expected/hoped for. My question is really if *
>* the solver does something smart when such equality constaints are present*
>* or if it would be worthwhile to merge the variables, either using a better*
>* model or by pre-processing. I assume that there in general can be *
>* round-off issues when performing this type of variable merging, but maybe*
>* that is tractable.*
Currently the glpk mip preprocessor performs only simple
transformations, in particular, it does not eliminate identical variables
defined by equality constraints like x = y.
I am implementing a new, more flexible version of the preprocessor common
for all three solvers (simplex, interior point, and branch-and-cut),
which will include processing such constraints. But I cannot say when
it will appear in glpk.