On zero-sum 6-flows of graphs

For a graph G, a zero-sum flow is an assignment of non-zero real numbers on the edges of G such that the total sum of all edges incident with any vertex of G is zero. A zero-surn k-flow for a graph G is a zero-sum now with labels from the set {+/- 1,..., +/- (k - 1)}. In this paper for a graph G, a necessary and sufficient condition for the existence of zero-sum flow is given. We conjecture that if G is a graph with a zero-sum flow, then G has a zero-sum 6-flow. It is shown that the conjecture is true for 2-edge connected bipartite graphs, and every r-regular graph with r even, r > 2, or r = 3. (C) 2009 Elsevier Inc. All rights reserved.