Re: [Help-glpk] Newbie question

[Andrew Makhorin]

> I am trying to write
> a model how to minimize the production cost 

> given the total 
> demand and production cost per production plant. 

> The thing is that I
> don't want have all production plants in use as it is now given my 
> model below.
> I would only like to start up a new plant when the demand of all 
> the cheaper production isn't suffient.

> I think my problem
> is a mix of different kind of problems and not a pure minimize 
> problem.

> Please, can someone 
> help me out on this? 

Probably you need to introduce binary variables:

var z{i in PLANTS}, binary;

where z[i] = 1 means that plant i is used, otherwise z[i] = 0. If plant
i is not used, it can produce nothing, therefore:

s.t. limits1{i in PLANTS} : p[i] >= z[i] * minlim[i];
s.t. limits2{i in PLANTS} : p[i] <= z[i] * maxlim[i];
/* for each plant within production limits */