How to add an if conditional statement in an integer program?

Question

I have an integer program with binary variables $x_{i,j,k}\in \{ 0,1 \}$.

For all $k$, I have that $\sum_i \sum_j =2$. I want to add a constraint that says if $x_{i_0,j,k}$ is equal to $1$ for some $i_0$ and a fixed $j$ and $k$, then $x_{i_1,j,k}$ must also equal to $1$ for some $i_1\neq i_0$ for the same fixed $j$ and $k$.

I'm unsure how to write this constraint, none of my ideas seem to get it right. Any ideas are welcomed and appreciated.

Answer 1

Here's one way do do it (which works even in the absence of the $\sum_i \sum_j x_{ijk} = 2$ constraints).

For each pair $(j,k)$, add a new integer variable $y_{jk}$, with the inequalities $y_{jk} \ge x_{ijk}$ for all $i$. Finally, add the constraint $$\sum_i x_{ijk} \ge 2y_{jk}. \quad (\star)$$ If there is no $i$ with $x_{ijk}=1$, then we can set $y_{jk}$ to $0$ without a problem and make $(\star)$ trivially satisfied. On the other hand, if there is at least one $i$ with $x_{ijk}=1$, then $y_{jk}$ must be $1$, and then $(\star)$ implies that there are at least two distinct $i$ with $x_{ijk}=1$.

Answer 2

Here is a way to do it without additional variables.

$\forall i_0,j,k, x_{i_0,j,k} \leq \sum_{i,i\neq i_0} x_{i,j,k} $

In a more general manner, whenever you have an "if A then B" in your head, with A and B taking values in $\{0,1\}$ and which you know how to express with linear expressions on your variables, $A \leq B$ usually does the trick. It is not the case for all logical constraints, some require non-linear constraints or additional variables.