A Dichotomy Theorem for Linear Time Homomorphism Orbit Counting in Bounded Degeneracy Graphs

Counting the number of homomorphisms of a pattern graph H in a large input graph G is a fundamental problem in computer science. There are myriad applications of this problem in databases, graph algorithms, and network science. Often, we need more than just the total count. Especially in large network analysis, we wish to compute, for each vertex v of G, the number of H-homomorphisms that v participates in. This problem is referred to as homomorphism orbit counting, as it relates to the orbits of vertices of H under its automorphisms. Given the need for fast algorithms for this problem, we study when near-linear time algorithms are possible. A natural restriction is to assume that the input graph G has bounded degeneracy, a commonly observed property in modern massive networks. Can we characterize the patterns H for which homomorphism orbit counting can be done in linear time? We discover a dichotomy theorem that resolves this problem. For pattern H, let l be the length of the longest induced path between any two vertices of the same orbit (under the automorphisms of H). If l <= 5, then H-homomorphism orbit counting can be done in linear time for bounded degeneracy graphs. If l > 5, then (assuming fine-grained complexity conjectures) there is no near-linear time algorithm for this problem. We build on existing work on dichotomy theorems for counting the total H-homomorphism count. Somewhat surprisingly, there exist (and we characterize) patterns H for which the total homomorphism count can be computed in linear time, but the corresponding orbit counting problem cannot be done in near-linear time.