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If you are looking for a way to ensure (in a MILP model) that a graph with $p$ nodes is connected, a common approach is to treat each edge as a pair of directed edges (adding flow variables $x_{ij}$ and $x_{ji}$ for each edge $(i,j)$, choose one node arbitrarily to have supply $p-1$ of a phantom commodity, and assign a demand of $1$ to every other node. You ...


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Try adding valid constraints $$ y_{i,j} \le \sum_{(i,k): k \in \tilde{V} \setminus \{j\}} y_{i,k} \quad\text{for $(i,j)$ such that $\hat{y}_i = 0$ and $j \in \tilde{V}\setminus\{i\}$} $$ that enforce the logical implications $$(y_{i,j} \land \lnot\hat{y}_i \land [j \in \tilde{V} \setminus\{i\}]) \implies \bigvee_{(i,k): k \in \tilde{V} \setminus \{j\}} y_{i,...


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