 # Boost :

Subject: Re: [boost] [BGL] Stoer–Wagner min-cut algorithm
From: Dan Jiang (danjiang_at_[hidden])
Date: 2010-07-05 14:47:00

Hi Daniel,
Thanks for the clarification.
What I really need to is to find source set of a min-cut between s(source) and t(target) in a directed graph, and hence a maximum closure of the graph. So, I need to find a s-t min-cut, not just any min-cut. Can Stoer-Wager's min-cut be forced to find s-t min-cut only (and thus has reduced time complexity)? If not then, I guess I'll have to stick with a max-flow algorithm.
Thanks,
Dan

> Date: Mon, 5 Jul 2010 13:51:41 -0400
> From: dtrebbien_at_[hidden]
> To: boost_at_[hidden]
> Subject: Re: [boost] [BGL] Stoer–Wagner min-cut algorithm
>
> > Hi Daniel,
> > I wonder if your min-cut algorithm can be used to find max flow, since
> > min-cut and max-flow are dual? I am currently using two max-flow
> > implementations in BOOST (i.e, Goldberg and Kolmogorov). What is the
> > complexity compared to the two max-flow implementations in BOOST. I am
> > looking for fastest max flow implementation available.
> > Another related question: does BOOST offer parallel implementation of any
> > max-flow algorithm?
> > Thanks
> > Dan
>
> Hi Dan,
>
> I should have been more precise. Given two vertices s and t where s is
> called the source and t is called the sink, the maximum flow from s to
> t is equal to the weight of a minimum s-t cut. Thus, a maximum flow
> algorithm can compute a minimum s-t cut of a graph. The Stoer–Wagner
> algorithm which I implemented calculates a min-cut of the graph, which
> is a slightly different problem.
>
> A cut of an undirected graph G = (V, E) is a partition of the vertices
> into two, non-empty sets X and Y = V - X. The weight w(X, Y) of a cut
> is the number of edges between X and Y if G is unweighted, or the sum
> of the weights of edges between X and Y if G is weighted. Given two
> vertices s and t, an s-t cut is a cut (X, Y) that separates s and t;
> that is, either s is in X and t is in Y or t is in X and s is in Y. A
> minimum s-t cut is an s-t cut of minimal weight.
>
> Historically, one way of computing a min-cut of a graph was to
> calculate the max-flow for every pair of vertices (s, t) and simply
> pick a minimum s-t cut of minimal weight. This would be a min-cut.
>
> I think that it is correct to say that if a min-cut is (X, Y), then
> for all x in X and y in Y, (X, Y) is also a minimum x-y cut with the
> max-flow from x to y and vice versa (because G is undirected) being
> the min-cut weight w(X, Y). I think that it is also correct to say
> that the least maximum flow between any two vertices of a graph is the
> weight of a min-cut.
>
> Intuitively speaking, because maximum flow is calculated between a
> given source and sink, then a maximum flow algorithm will be better
> than an min-cut algorithm, which has to consider all pairs of
> vertices. So, if you are given two vertices s and t and you want to
> know the maximum flow from s to t, you should use a maximum flow
> algorithm. Also, a min-cut (X, Y) of G as returned by the Stoer–Wagner
> algorithm might not be a minimum s-t cut; both s and t could be in X
> or Y.
>
> As far as parallel implementations of maximum flow algorithms, I do
> not know whether Parallel BGL offers parallel maximum flow algorithm
> implementations.
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