Friday, 29 January 2010

co.combinatorics - To what degree do min-cuts specify the cut function of a graph?

Given an unweighted graph G=(V,E), let the cut function on this graph be defined to be:
C:2VrightarrowmathbbZ such that:
CG(S)=|(u,v)inE:uinSwedgevnotinS|



For any two vertices i,jinV, let the (i,j) min-cut in a graph G be:
alphai,j(G)=minSsubsetV:iinS,jnotinSCG(S)


Now, suppose we have two unweighted graphs on the same vertex set, G=(V,E) and H=(V,E) such that they are identical with respect to all (i,j) min-cuts:
foralli,jinV,alphai,j(G)=alphai,j(H)

How much can H and G differ with respect to their cuts? That is, how large can the following quantity be:
Delta(H,G)=maxSsubsetV|CG(S)CH(S)|



Note that if the graphs are allowed to be weighted (or to be multigraphs), then for any G, there is a tree T that agrees with G on all min-cuts (A Gomory-Hu tree). But I am interested in the case of unweighted graphs...

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