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Group twin coloring of graphs. (English) Zbl 1401.05107
Summary: For a given graph $$G$$, the least integer $$k\geq 2$$ such that for every abelian group $${\mathcal G}$$ of order $$k$$ there exists a proper edge labeling $$f:E(G)\to{\mathcal G}$$ so that $$\sum_{x\in N(u)}f(xu)\neq\sum_{x\in N(v)}f(xv)$$ for each edge $$uv\in E(G)$$ is called the group twin chromatic index of $$G$$ and denoted by $$\chi^\prime_g(G)$$. This graph invariant is related to a few well-known problems in the field of neighbor distinguishing graph colorings. We conjecture that $$\chi^\prime_g(G)\leq\Delta(G)+3$$ for all graphs without isolated edges, where $$\Delta(G)$$ is the maximum degree of $$G$$, and provide an infinite family of connected graph (trees) for which the equality holds. We prove that this conjecture is valid for all trees, and then apply this result as the base case for proving a general upper bound for all graphs $$G$$ without isolated edges: $$\chi^\prime_g(G)\leq 2(\Delta(G)+ \text{col}(G))-5$$, where col$$(G)$$ denotes the coloring number of $$G$$. This improves the best known upper bound known previously only for the case of cyclic groups $$\mathbb{Z}_k$$.
##### MSC:
 05C15 Coloring of graphs and hypergraphs
##### Keywords:
abelian group; twin edge coloring
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