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Let's suppose we have a weighted and connected graph. We can easily find the minimum spanning tree for this graph. But let's say we want to "force" a certain edge $e$ to be in the MST. For doing so, we are allowed to remove some edges from the graph. But I don't want to remove too many edges. Is there an efficient algorithm that can find the minimum number of edges to remove before $e$ becomes a member of MST?

I am particularly interested in that minimum value, and not the edges to remove.

I think there is a way to find a spanning tree that contains $e$. All we have to do is to insert a dummy node between the two nodes that are connected by $e$. But I doubt this helps. Any hint would be appreciated.

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  • $\begingroup$ Do you want to make sure that $e$ is in all or in some MST? $\endgroup$
    – Pål GD
    Jan 14 at 12:30
  • $\begingroup$ @PålGD just some MST would do it. $\endgroup$ Jan 14 at 12:42
  • $\begingroup$ Which property must $e$ have to never be in any MST? $\endgroup$
    – Pål GD
    Jan 14 at 12:43
  • $\begingroup$ It must be the heaviest edge in some cycle. Am I right? @PålGD $\endgroup$ Jan 14 at 12:47

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An edge $uv$ is in some MST if and only if it is a minimum-weight edge of some cut. So we need to find a $uv$-cut with a minimum number of edges whose weight is strictly less than $w(uv)$. Once those edges are removed, $uv$ will be in some MST.

Algorithmically, any min $st$-cut algorithm, applied on the subgraph of edges of weight less than $w(uv)$, will give us the answer. For instance you can use a max $uv$-flow algorithm to find the cut, if you don't know fancier algorithms.

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  • $\begingroup$ Your first paragraph is good, the second paragraph does a leap of faith and I don't think it's completely sound. $\endgroup$
    – Pål GD
    Jan 14 at 14:27
  • $\begingroup$ $K_{3,2}$ is a counter-example if you let one of the degree-2 vertices have both its edges weight 2 and the remaining edges have weight 1. Let $e=uv$ be an edge with weight 2. A minimum $uv$-cut is now for example edges with weight $2,1,1$. However, $e$ is actually already in an MST. $\endgroup$
    – Pål GD
    Jan 14 at 14:29
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    $\begingroup$ No, if $v$ is the degree-2 vertex whose incident edges have weight 2, the min $uv$-cut is $\delta(v)$, which is empty, in the graph of edges with weight $< 2$. $\endgroup$
    – Guyslain
    Jan 14 at 16:17

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