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Improper coloring of sparse graphs with a given girth. I: \((0,1)\)-colorings of triangle-free graphs. (English) Zbl 1297.05083
Summary: A graph \(G\) is \((0, 1)\)-colorable if \(V(G)\) can be partitioned into two sets \(V_0\) and \(V_1\) so that \(G [V_0]\) is an independent set and \(G [V_1]\) has maximum degree at most 1. The problem of verifying whether a graph is \((0, 1)\)-colorable is NP-complete even in the class of planar graphs of girth 9. The maximum average degree, \(\operatorname{mad}(G)\), of a graph \(G\) is the maximum of \(\frac{2 | E(H) |}{| V(H) |}\) over all subgraphs \(H\) of \(G\). It was proved recently that every graph \(G\) with \(\operatorname{mad}(G) \leq \frac{12}{5}\) is \((0, 1)\)-colorable, and this is sharp. This yields that every planar graph with girth at least 12 is \((0, 1)\)-colorable. Let \(F(g)\) denote the supremum of \(a\) such that for some constant \(b_g\) every graph \(G\) with girth \(g\) and \(| E(H) | \leq a | V(H) | + b_g\) for every \(H \subseteq G\) is \((0, 1)\)-colorable. By the above, \(F(3) = 1.2\).
We find the exact value for \(F(4)\) and \(F(5)\): \(F(4) = F(5) = \frac{11}{9}\). In fact, we also find the best possible values of \(b_4\) and \(b_5\): every triangle-free graph \(G\) with \(| E(H) | < \frac{11 | V(H) | + 5}{9}\) for every \(H \subseteq G\) is \((0, 1)\)-colorable, and there are infinitely many not \((0, 1)\)-colorable graphs \(G\) with girth 5, \(| E(G) | = \frac{11 | V(G) | + 5}{9}\) and \(| E(H) | < \frac{11 | V(H) | + 5}{9}\) for every proper subgraph \(H\) of \(G\). A corollary of our result is that every planar graph of girth 11 is \((0, 1)\)-colorable. This answers a half of a question by P. Dorbec et al. [J. Graph Theory 75, No. 2, 191–202 (2014; Zbl 1280.05040)]. In a companion paper, we show that for every \(g\), \(F(g) \leq 1.25\) and resolve some similar problems for the so-called \((j, k)\)-colorings generalizing \((0, 1)\)-colorings.

MSC:
05C15 Coloring of graphs and hypergraphs
05C42 Density (toughness, etc.)
05C10 Planar graphs; geometric and topological aspects of graph theory
05C35 Extremal problems in graph theory
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