On {a,b}-edge-weightings of bipartite graphs with odd a,b

International audienceFor any S⊂ℤ we say that a graph G has the S-property if there exists an S-edge-weighting w:E(G)→S such that for any pair of adjacent vertices u,v we have Σ_{e∈E(v)} w(e) ≠ Σ_{e∈E(u)} w(e), where E(v) and E(u) are the sets of edges incident to v and u respectively. This work focuses on {a,a+2}-edge-weightings where a∈ℤ is odd. We show that a 2-connected bipartite graph has the {a,a+2}-property if and only if it is not a so-called odd multi-cactus. In the case of trees, we show that only one case is pathological. That is, we show that all trees have the {a,a+2}-property for odd a≠−1, while there is an easy characterization of trees without the {−1,1}-property

Graph factors modulo k

We prove a general result on graph factors modulo k . A special case says that, for each natural number k , every (12k−7)-edge-connected graph with an even number of vertices contains a spanning subgraph in which each vertex has degree congruent to k modulo 2k