TY  - JOUR
A1  - Göbel, Andreas
A1  - Lagodzinski, Gregor J. A.
A1  - Seidel, Karen
T1  - Counting homomorphisms to trees modulo a prime
JF  - ACM transactions on computation theory : TOCT / Association for Computing Machinery
N2  - Many important graph-theoretic notions can be encoded as counting graph homomorphism problems, such as partition functions in statistical physics, in particular independent sets and colourings. In this article, we study the complexity of #(p) HOMSTOH, the problem of counting graph homomorphisms from an input graph to a graph H modulo a prime number p. Dyer and Greenhill proved a dichotomy stating that the tractability of non-modular counting graph homomorphisms depends on the structure of the target graph. Many intractable cases in non-modular counting become tractable in modular counting due to the common phenomenon of cancellation. In subsequent studies on counting modulo 2, however, the influence of the structure of H on the tractability was shown to persist, which yields similar dichotomies. <br /> Our main result states that for every tree H and every prime p the problem #pHOMSTOH is either polynomial time computable or #P-p-complete. This relates to the conjecture of Faben and Jerrum stating that this dichotomy holds for every graph H when counting modulo 2. In contrast to previous results on modular counting, the tractable cases of #pHOMSTOH are essentially the same for all values of the modulo when H is a tree. To prove this result, we study the structural properties of a homomorphism. As an important interim result, our study yields a dichotomy for the problem of counting weighted independent sets in a bipartite graph modulo some prime p. These results are the first suggesting that such dichotomies hold not only for the modulo 2 case but also for the modular counting functions of all primes p.
KW  - Graph homomorphisms
KW  - modular counting
KW  - complexity dichotomy
Y1  - 2021
U6  - https://doi.org/10.1145/3460958
SN  - 1942-3454
SN  - 1942-3462
VL  - 13
IS  - 3
SP  - 1
EP  - 33
PB  - Association for Computing Machinery
CY  - New York
ER  -