A Dichotomy for Real Weighted Holant Problems

Abstract

Holant is a framework of counting characterized by local constraints. It is closely related to other well-studied frameworks such as the counting constraint satisfaction problem (#CSP) and graph homomorphism. An effective dichotomy for such frameworks can immediately settle the complexity of all combinatorial problems expressible in that framework. Both #CSP and graph homomorphism can be viewed as subfamilies of Holant with the additional assumption that the equality constraints are always available. Other subfamilies of Holant such as Holant* and Holantc problems, in which we assume some specific sets of constraints to be freely available, were also studied. The Holant framework becomes more expressive and contains more interesting tractable cases with less or no freely available constraint functions, while, on the other hand, it also becomes more challenging to obtain a complete characterization of its time complexity. Recently, a complexity dichotomy for a variety of subfamilies of Holant such as #CSP, graph homomorphism, Holant*, and Holantc for Boolean domain was proved. In this paper, we prove a dichotomy for the general Holant framework where all the constraints are real symmetric functions on Boolean inputs. This setting already captures most of the interesting combinatorial counting problems defined by local constraints, such as (perfect) matching. This is the first time a dichotomy is obtained for general Holant problems without any auxiliary functions. One benefit of working with the Holant framework is some powerful new reduction technique such as the holographic reduction. Along the proof of our dichotomy, we introduce a new reduction technique, namely realizing a constraint function by approximating it. This new technique is employed in our proof in a situation where it seems that all previous reduction techniques fail; thus, this new idea of reduction might also be of independent interest. Besides proving a dichotomy and developing a new technique, we also obtained some interesting by-products. We prove a dichotomy for #CSP, restricting to instances where each variable appears a multiple of d times for any d. We also prove that counting the number of Eulerian orientations on 2k-regular graphs is #P-hard for any $${k \geq 2}$$k?2.