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We are given a flow network and two nodes $u$ and $v$. We want to create an algorithm that tells us whether or not there is a minimum s-t cut so that $u$ belongs to the same side of the cut as the source node $s$ and $v$ belongs to the same side of the cut as the sink node $t$.

My initial idea was to run Ford-Fulkerson to get the residual graph $G_f$ at the end. Then use that to find all nodes reachable from $s$ in $G_f$ (call this set $A$) then $B=V-A$ where $V$ is the vertex set. Then $(A,B)$ is a min s-t cut and if $u \in A, v \in B$ output true, else false.

But since there may be more than one min cut I'm a bit stuck.

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  • $\begingroup$ I'm not fully confident, but the following should work. Essentially, you need to find a min-cut between sets $\{s,u\}$ and $\{t,v\}$. For this, find a max flow with sources $s$ and $u$ and sinks $t$ and $v$. After that, check that the value of this flow matches the value of the min cut between $s$ and $t$. $\endgroup$
    – user114966
    Mar 20, 2021 at 23:46
  • $\begingroup$ @Dmitry $u$ could have incoming edges and $t$ could have outgoing edges. I'm not sure how to run Ford-Fulkerson on something like that. $\endgroup$
    – LTM
    Mar 22, 2021 at 23:38

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The easy solution is to add an edge between $s$ and $u$ with infinite weight and $t$ and $v$ with infinite weight.

Note though that this doesn't mean that $s$ and $u$ will be connected in $G[A]$.

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  • $\begingroup$ I agree that if the max-flow of the new graph changes then the vertices were part of every cut. But, if the max-flow of the new graph stays the same, nodes u,v could've been part of a min-cut ( in such a case, there were at-least two min-cuts ), but not necessarily so; how do we verify this? $\endgroup$
    – flamel12
    Jul 9, 2023 at 13:24
  • $\begingroup$ OP asks for a minimum cut. $\endgroup$
    – Pål GD
    Jul 10, 2023 at 11:19
  • $\begingroup$ For some vertices $u,v$ we want to find if there exists a min-cut $(S,T) $ s.t. $ u \in S $ and $ v \in T $. Why running FF method on the new graph you proposed does satsify those vertices membership in $(S,T)$? ( I don't understand the correctness of your algorithm ) $\endgroup$
    – flamel12
    Jul 10, 2023 at 11:52
  • $\begingroup$ Any edge across a cut is at capacity in the residual graph. If a minimum cut does not contain neither $su$ nor $vt$, then the cut will remain at capacity. $\endgroup$
    – Pål GD
    Jul 10, 2023 at 12:09

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