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Suppose we have $S=\{1,2,\ldots,n\}$, a binary variable $x$ and an integer $p$. I would like to model the following constraint using integer linear programming:

If $x = 1$, then there must exists a unique $i\in S$ such that: $x_j=1$ for all $j\in\{i,i+1,\ldots,i+p-1\}$.

How can I do this?

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  • $\begingroup$ Is $x$ a scalar or a vector? $\endgroup$ Apr 11, 2017 at 9:07
  • $\begingroup$ $x$ is a scalar $\endgroup$
    – Zir
    Apr 12, 2017 at 1:24

1 Answer 1

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Assuming $x_j$ are binary.

The additional binary variable $y_i$ equals one if and only if when all $j$ are 1.

$$ \sum\limits_{j=i}^{i+p-1}x_j - p+1 \leq y_i \leq \frac{1}{p}\sum\limits_{j=i}^{i+p-1}x_j $$

Then to have a unique $y_i$ equals one you add $$ x \leq \sum_{i=1}^n y_i \leq (n-1)(1-x)+1 $$

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  • $\begingroup$ Can be done hopefully better (tighter) if the objective is known. $\endgroup$
    – Eugene
    Apr 10, 2017 at 2:38

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