Description

In this model we drive the scalar c down towards zero,
making for a poorly scaled model.  For nice c vals, the model
is unbounded.  As c decreases, x2 increases in proportion.
Eventually, either GAMS or the solver will squeeze the constant c
out of the matrix and the model becomes solvable.

Contributor: Michael Bussieck

Small Model of Type : LP

Category : GAMS Test library

Main file : lp10.gms

$title Poorly-scaled model - small matrix entry implies large x (LP10,SEQ=118)

$ontext
In this model we drive the scalar c down towards zero,
making for a poorly scaled model.  For nice c vals, the model
is unbounded.  As c decreases, x2 increases in proportion.
Eventually, either GAMS or the solver will squeeze the constant c
out of the matrix and the model becomes solvable.

Contributor: Michael Bussieck
$offtext

$if not set MTYPE       $set MTYPE lp
$if not set solvedasmcp $set solvedasmcp 0

variable x1,x2,x99,x100,z;
equation e1,e2;
positive variable x99, x100;

scalar c /1/;
e1.. z =e= x1 + x99;
e2.. x1 + c*x2 + x99 =e= x100;

model m /all/;
option limrow=0, limcol=0;
m.modelstat = %modelstat.Unbounded%;
scalar loopCount / 0 /;
while {(m.modelstat = %modelstat.Unbounded%) or (m.modelstat = %modelstat.Unbounded-NoSolution%) or
    (m.modelstat = %modelstat.Infeasible% and %solvedasmcp%=1) or (m.modelstat = %modelstat.Infeasible-NoSolution%),
  loopCount = loopCount + 1;
  abort$[loopCount > 100] 'Too many trips!';
* Initialize all variables and marginals to default
   z.l = 0; x1.l = 0; x2.l = 0; x99.l = 0; x100.l = 0;
   e1.m = 0; e2.m = 0;
   c = c/10;
   solve m min z using %MTYPE%;
};
abort$(c>1e-5) 'solver %system.lp% declared model optimal too early';
$exit
file fx / 'lp10.txt' /; fx.ap=1; fx.nz = 0; fx.nw = 15;
put fx  '%system.lp%':10 ' number of solves: ' m.number:2:0;
fx.nr=2;
put ' final c: '   c;