Graph homomorphisms with complex values: a dichotomy theorem

  • Authors:
  • Jin-Yi Cai;Xi Chen;Pinyan Lu

  • Affiliations:
  • University of Wisconsin-Madison;University of Southern California;Microsoft Research Asia

  • Venue:
  • ICALP'10 Proceedings of the 37th international colloquium conference on Automata, languages and programming
  • Year:
  • 2010

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Abstract

Graph homomorphism problem has been studied intensively. Given an m × m symmetric matrix A, the graph homomorphism function ZA(G) is defined as ZA(G) = σξ:V →[m] (u,v)∈E ΠAξ(u),ξ(v), where G = (V,E) is any undirected graph. The function ZA(G) can encode many interesting graph properties, including counting vertex covers and k-colorings. We study the computational complexity of ZA(G), for arbitrary complex valued symmetric matrices A. Building on the work by Dyer and Greenhill [1], Bulatov and Grohe [2], and especially the recent beautiful work by Goldberg, Grohe, Jerrum and Thurley [3], we prove a complete dichotomy theorem for this problem.