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## Re: [Help-glpk] lpx_integer confusion

**From**: |
Andrew Makhorin |

**Subject**: |
Re: [Help-glpk] lpx_integer confusion |

**Date**: |
Tue, 13 Jul 2004 02:08:26 +0400 |

>*Hello, I am attempting to solve a binary*
>*integer programming problem. I call the lpx_integer function, but*
>*first I call the lpx_simplex for an initial optimal solution. For*
>*some reason, occasionally, the lpx_simplex function seems to get some*
>* information*
>*in it that makes all further calls to it useless, even when the*
>* coefficients*
>*for A and f have changed. The messages I continually get from*
>* lpx_simplex*
>*after this point are:*
>
>*spx_invert: trying to factorize the*
>*basis using threshold tolerance 0.3*
>*spx_invert: trying to factorize the*
>*basis using threshold tolerance 0.7*
>*spx_invert: the basis matrix is singular*
>*spx_simplex: initial basis is singular*
This means that the initial basis (defined by current statuses of rows
and columns) is singular and therefore cannot be used to start/continue
the search. If you do not intend to provide an initial basis, call
lpx_std_basis or lpx_adv_basis (before lpx_simplex) to construct a valid
initial basis. This might happen if you solved the problem, changed
constraint coefficients at basic variables, and tried to re-optimize it.
On exit lpx_simplex keeps statuses of rows and columns in LPX and uses
them for re-optimization.
>*So, if lpx_simplex does not return LPX_E_OK,*
>*I call lpx_interior instead. lpx_interior seems to do fine, and I*
>*get the following response:*
lpx_integer requires an optimal *basic* solution while lpx_interior
computes an *interior-point* solution.
Andrew Makhorin