Optimal solution is solver-specific

Hello,

I am facing the following strange problem. I have a simple cost-minimization problem,

* Defining in=line comments
$inLineCom # #

Set
    i 'type of input' /water, fertilizer, energy, labor, land/;
    
Variable
    Z;
    
POsitive Variables
     X(i);
    
    
Equations
    cost, con1, con2;
    
    cost.. Z =e= sum(i,X(i)); 
    
    con1.. X('water')+ X('fertilizer')+X('energy') =e= 2;
    con2.. X('water')+ X('fertilizer')+X('energy')+X('land') =g= 2;
    
Model base /all/;


Solve base min Z using lp;

By default, the solver is set to CPLEX and the optimal solution is energy=2. However, if i change the solver, eg

Option lp=Xpress;

then I obtain a different solution, water=2. The same arises if I set the Xpress optimizer.

Why this happens?

I run GAMS 30.0.3 on windows 10(64bit)

Alternative optimal solutions? Both solutions have objective 2. So you get different (primal) solutions depending on pure chance. This is not uncommon. Just search the internet for “degenerate linear programs”.

-Michael

thanks for the recommendation!