# relation beetwen variables

 0 Hello! I need to put in relations couples of variables. This seems to make my model not-linear. My model manages two groups of variables (all of them boolean, 0 or 1). Part of objective depends on couples of variables coming from the two different groups are both equal to 1. So I check this multiply couples of variables each other and for a constant. The result is 0 (zero) or the constant. But this part of model makes it not-linear. Generally speaking how can I keep model linear in this case? thanks asked 17 Feb '18, 09:23 cla68 11●1 accept rate: 0%

 0 Imagine you have the boolean variables x[i] and y[j], then you can have another set of boolean variables z[i][j] where z[i][j] takes value 1 iff both x[i] and y[j] have value 1: $$2 z_{ij} \geq x_i + y_j$$, $$\sum_j x_{ij} \leq x_i$$, $$\sum_i x_{ij} \leq y_j$$. answered 19 Feb '18, 12:49 Alberto Santini 1●1 accept rate: 0% Thanks Alberto. I suppose you are Italian as I am. Your suggestion is ok, but it does not solve the main problem. That's because I did not explain it properly. Basically my problem is similar to classic example of transport optimization, but from/to sources/destinations are not defined because are variables of the problem. The problem has two targets (but together in the same objective as sum): one is optimization of choice sources/destination (based on an economic target, minimize the total cost of products supplied by different suppliers); at the same time part of the objective (minimize total cost) depends on transportation cost, which depends on couples of sources/destination. So, first part of problem is linear, but second part needs to compare the source/destination in order to calculate transportation cost, and that makes the problem not-linear. Maybe I can complete re-formulate problem (at the moment I choose source/destination with binary variables) but I do not find the right way. I cannot exclude there isn't a linear formulation...Thanks in advance for any further suggestion (19 Feb '18, 15:06) cla68
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