# MIP: If integer variable $>0$ it should be equal to other integer variables $>0$

I have an MIP problem where $$n$$ different types of cars are delivering packages. Sometimes multiple types of cars are required to go to a single location. For example if car $$1$$ makes two deliveries to location $$z$$, car $$2$$ needs to make an equal amount of deliveries as well.

In other words: if an integer variable $$x_n$$ is larger than $$0$$ it should be equal to all $$x_n$$ for all $$x_n$$ larger than $$0$$

How do I capture this in constraints in linear programming?

I think I need to create a new decision variable $$b_i$$ that is $$1$$ if a car makes $$1$$ or more deliveries, and $$0$$ if there are no deliveries. From there on I need to somehow set the $$x_n$$ values equal. But how?

• It sounds like you are just saying $x_n = x_m$ for all $n$, $m$ -- is that correct? If one of them equals 0 then they all must equal 0, and if one of them equals something positive then they all must equal the same positive number? Nov 12, 2019 at 13:36
• Thanks, but that's incorrect. If one of them equals zero, others must still be able to be positive. However, all positive $x_n$ have to be equal.
– Tim
Nov 12, 2019 at 13:41
• Ah. Sorry. I missed that. Thanks for the clarification, and welcome to OR.SE! Nov 12, 2019 at 13:42

Let $$b_n$$ be a binary indicator variable, and let integer variable $$y$$ be the common value of the positive $$x_n$$. Then you want to enforce $$x_n=b_n y$$, which you can linearize using the formulation given here.