Minconvex Factors of Prescribed Size in Graphs∗ Nicola Apollonio ,y Andr´as Seb}o z January 24, 2006 Abstract We provide a polynomial algorithm that determines for any given undi- rected graph, positive integer k and a separable convex function on the degree sequences, k edges that minimize the function. The motivation and at the same time the main application of the results is the problem of finding a subset of k vertices in a line graph, that cover as many edges as possible (we called it maxfix cover even though k is part of the input), generalizing the vertex cover problem for line graphs, equivalent to the maximum matching problem in graphs. The usual improving paths or walks for factorization problems turn into edge-disjoint unions of pairs of such paths for this problem. We also show several alternatives for reducing the problem to b-matching problems { lead- ing to generalizations and variations. The algorithms we suggest also work if for any subset of vertices, upper, lower bound constraints or parity constraints are given. In particular max- imum (or minimum) weight b-matchings of given size can be determined in polynomial time, combinatorially, in more than one way, for arbitrary b. Fur- thermore, we provide an alternative "gadget reduction" method to solve the problems which leads to further generalizations, provides ideas to alter the ∗A a preliminary shortened version with part of the results (and a small part of the proofs) has appeared in the IPCO 2004 volume: LNCS 3064 (D. Bienstock and G. Nemhauser, eds.). yUniversity \La Sapienza" di Roma, supported by the European project ADONET while the author visited Laboratoire Leibniz (IMAG), Grenoble,
[email protected] zCNRS, Leibniz-IMAG, Grenoble, France,
[email protected] 1 usual optimization problems themselves, and solves the arising problems in polynomial time.