Splitting off edges between two subsets preserving the edge-connectivity of the graph Original Research Article

Splitting off a pair of edges su, sv in a graph G means replacing these two edges by a new edge uv. This operation is well known in graph theory. Let G=(V+s,E+F) be a graph which is k-edge-connected in V and suppose that |F| is even. Here F denotes the set of edges incident with s. Lovász (Combinatorial Problems and Exercises, North-Holland, Amsterdam, 1979) proved that if k⩾2 then the edges in F can be split off in pairs preserving the k-edge-connectivity in V. This result was recently extended to the case where a bipartition R∪Q=V is given and every split edge must connect R and Q (SIAM J. Discrete Math. 12 (2) (1999) 160). In this paper, we investigate an even more general problem, where two disjoint subsets R,Q⊂V are given and the goal is to split off (the largest possible subset of) the edges of F preserving k-edge-connectivity in V in such a way that every split edge incident with a vertex from R has the other end-vertex in Q. Motivated by connectivity augmentation problems, we introduce another extension, the so-called split completion version of our problem. Here, the smallest set F∗ of edges incident to s has to be found for which all the edges of F+F∗ can be split off in the augmented graph G=(V+s, E+F+F∗) preserving k-edge-connectivity and in such a way that every split edge incident with a vertex from R has the other end-vertex in Q. We solve each of the above extensions when k is even: we give min–max formulae and polynomial algorithms to find the optima. For the case when k is odd we show how to find a solution to the split completion problem using at most two edges more than the optimum.