English

Homotopy types of Hom complexes of graph homomorphisms whose codomains are cycles

Combinatorics 2025-09-08 v2 Algebraic Topology

Abstract

For simple graphs GG and HH, the Hom complex Hom(G,H)\mathrm{Hom}(G,H) is a polyhedral complex whose vertices are the graph homomorphisms GHG\to H and whose edges connect the pairs of homomorphisms which differ in a single vertex of GG. Hom complexes play an important role in an algebro-topological approach to the graph coloring problem. It is known that Hom(G,H)\mathrm{Hom}(G,H) is homotopy equivalent to a disjoint union of points and circles when both GG and HH are cycles. We generalize this known result by showing that the same holds whenever GG is connected and HH is a cycle. To this end, we explicitly construct the universal cover of each connected component of Hom(G,H)\mathrm{Hom}(G,H) and prove that it is contractible. Additionally, we provide a simple criterion to determine whether the connected component containing a given homomorphism is homotopy equivalent to a point or circle.

Keywords

Cite

@article{arxiv.2408.04802,
  title  = {Homotopy types of Hom complexes of graph homomorphisms whose codomains are cycles},
  author = {Soichiro Fujii and Yuni Iwamasa and Kei Kimura and Yuta Nozaki and Akira Suzuki},
  journal= {arXiv preprint arXiv:2408.04802},
  year   = {2025}
}

Comments

13 pages, 5 figures