
Any graph $G$ with chromatic number $k$ can be constructed by iteratively performing certain graph operations on a sequence of graphs starting with $K_k$, resulting in a variety of Hajós-type constructions for $G$. Finding such constructions for a given graph or family of graphs is a challenging task. We show that the basic steps in these Hajós-type constructions frequently result in the presence of an $S^1$-wedge summand in the neighborhood complex of the resulting graph. Our results imply that for a graph $G$ with a highly-connected neighborhood complex, the end behavior of the construction sequence is quite restricted, and we investigate these restrictions in detail. We also introduce two graph construction algorithms based on different Hajós-type constructions and conduct computational experiments using these.
neighborhood complex, Coloring of graphs and hypergraphs, Betti number, Graph algorithms (graph-theoretic aspects), FOS: Mathematics, Algebraic Topology (math.AT), Combinatorial aspects of simplicial complexes, Combinatorics (math.CO), Hajós construction
neighborhood complex, Coloring of graphs and hypergraphs, Betti number, Graph algorithms (graph-theoretic aspects), FOS: Mathematics, Algebraic Topology (math.AT), Combinatorial aspects of simplicial complexes, Combinatorics (math.CO), Hajós construction
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