My problem is related to edge and vertex cuts with a little twist.

Given a graph $G$ and two vertexes $u$ and $v$. I want to find a set of vertexes $S \subset V$ that disconnects $u$ and $v$ such that the induced subgraph $G[S]$ has minimal number of edges.

Consider the following graph:

enter image description here

The red edges are the minimal edge cut. The blue vertices are the minimal vertex cut. The green vertices are a vertex cut such that the induced subgraph has the least number of edges. I try to find the green cut. In fact, the number of vertices is not required to be minimal as long as the number of edges in $G[S]$ are minimal.

Is there an algorithm in $P$ to solve this problem? Do you have an idea how to approach the problem?


1 Answer 1


An independent set that disconnects its graph is called an "independent cut", graphs that contain an independent cut are called "fragile graphs", and recognizing fragile graphs is known NP-complete [C. deFigueiredo, S. Klein, "NP-completeness of multipartite cutset testing", Congr. Numer. 119 (1996) 217–222, as cited by Guantao Chen and Xingxing Yu, "A note on fragile graphs", Discrete Math. 2002, https://doi.org/10.1016/S0012-365X(01)00226-6]. So your problem is hard even in the special case of zero edges.

  • 1
    $\begingroup$ Brilliant, thank you! My take away is: in case of a fragile graph, an efficient algorithm that solves my problem above would find an independent cut. Thus, such an algorithm cannot exist unless P = NP. $\endgroup$
    – Marian
    Commented May 8, 2021 at 21:23

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