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I have a problem that I'm struggling to solve or even name, I'd really appreciate any help or pointer to potential existing solutions.

Suppose there is a connected graph $G$ and we are trying to find a spanning tree $M^*$ with the following property. Let $D_M(v)$ be the degree of node $v$ in an arbitrary tree $M$. The solution $M^*$ satisfies:

$M^* = \arg \min_M \sum_{(ij)\in M} C_{ij}$ where $C_{ij} = c_{ij} + f(D_M(i),D_M(j))$ and is spanning ($V\subset M$).

Thus, this can be viewed as a "MST" where the cost of any edge depends on the degrees of the conneted nodes.

For the main case that I'm interested in, let

$$f = \beta \cdot \mathbf{1}\{\max(\text{deg}(i), \text{deg}(j)) > k \},$$

so there's essentially a cost $\beta$ for having an edge coming out of a node with more than $k$ connections. If I can solve this for $k=2$ I'd be very happy.

Added context: This problem is formulated to model a telecommunication network where the network planner tries to connect a hub to nodes. The cost $\beta$ captures "relay" cost: the network incurs $\beta$ if a path is split at a node (e.g. a Y-path would cost more than an L-path, all else equal). For intuition, $\beta \to \infty$ would eventually have the solution be a Hamiltonian path (as pointed out below, we only need $\beta >> |V|$. We want to solve this problem for a general $\beta$.

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  • $\begingroup$ Thanks! The problem is formulated from a practical problem. I added context. $\endgroup$
    – quanecon
    May 8, 2023 at 13:39
  • $\begingroup$ Are there any bounds on the value of $\beta$? If not, then we can assume $\beta \gg |V|$. If yes, please state that. $\endgroup$ May 8, 2023 at 21:37
  • $\begingroup$ $\beta$ is an exogenously given number, which could be any arbitrary value. Thanks for pointing out that $\beta >> |V|$ yields a special case! $\endgroup$
    – quanecon
    May 9, 2023 at 0:08

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The Hamiltonian Path problem on undirected graphs can be reduced to your problem for $k = 2$ and $\beta \gg |V|$. Since the Hamiltonian Path problem is $\mathsf{NP}$-hard, your problem is $\mathsf{NP}$-hard as well.

For the proof of the above statement, you just need to observe the fact that if any spanning tree has all vertices of degree $\leq2$ then it is a hamiltonian path and vice-versa.

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  • $\begingroup$ Thanks, that makes sense. Unfortunately in the setting above we're interested in interior solutions where the network planner would like to trade off the number of splits (which incurs $\beta$) and the intrinsic edge cost ($c_{ij}$), so finding a Hamiltonian Path where there is no split would be too restrictive. $\endgroup$
    – quanecon
    May 8, 2023 at 13:47
  • $\begingroup$ @quanecon Aren't you looking for a spanning tree: connected graph that spans all vertices? $\endgroup$ May 8, 2023 at 14:11
  • $\begingroup$ Yes! A spanning tree, where there are penalties for "splits". I added that wording. Thanks. $\endgroup$
    – quanecon
    May 8, 2023 at 14:20

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