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The 3-flow conjecture, factors modulo \(k\), and the 1-2-3-conjecture. (English) Zbl 1348.05085

Summary: Let \(k\) be an odd natural number \(\geq 5\), and let \(G\) be a \((6k-7)\)-edge-connected graph of bipartite index at least \(k-1\). Then, for each mapping \(f : V(G) \to \mathbb{N}\), \(G\) has a subgraph \(H\) such that each vertex \(v\) has \(H\)-degree \(f(v)\) modulo \(k\). We apply this to prove that, if \(c\colon V(G)\to \mathbb{Z}_k\) is a proper vertex-coloring of a graph \(G\) of chromatic number \(k \geq 5\) or \(k-1 \geq 6\), then each edge of \(G\) can be assigned a weight 1 or 2 such that each weighted vertex-degree of \(G\) is congruent to \(c\) modulo \(k\). Consequently, each nonbipartite \((6k-7)\)-edge-connected graph of chromatic number at most \(k\) (where \(k\) is any odd natural number \(\geq 3\)) has an edge-weighting with weights 1,2 such that neighboring vertices have distinct weighted degrees (even after reducing these weighted degrees modulo \(k\)). We characterize completely the bipartite graph having an edge-weighting with weights 1,2 such that neighboring vertices have distinct weighted degrees. In particular, that problem belongs to P while it is NP-complete for nonbipartite graphs. The characterization also implies that every 3-edge-connected bipartite graph with at least 3 vertices has such an edge-labeling, and so does every simple bipartite graph of minimum degree at least 3.

MSC:

05C15 Coloring of graphs and hypergraphs
05C60 Isomorphism problems in graph theory (reconstruction conjecture, etc.) and homomorphisms (subgraph embedding, etc.)
05C70 Edge subsets with special properties (factorization, matching, partitioning, covering and packing, etc.)
Full Text: DOI

References:

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