A Holant Dichotomy: Is the FKT Algorithm Universal?

We prove a complexity dichotomy for complex-weighted Holant problems with an arbitrary set of symmetric constraint functions on Boolean variables. In the study of counting complexity, such as #CSP, there are problems which are #P-hard over general graphs but P-time solvable over planar graphs. A recurring theme has been that a holographic reduction [36] to FKT precisely captures these problems. This dichotomy answers the question: Is this a universal strategy? Surprisingly, we discover new planar tractable problems in the Holant framework (which generalizes #CSP) that are not expressible by a holographic reduction to FKT. In particular, the putative form of a dichotomy for planar Holant problems is false. Nevertheless, we prove a dichotomy for #CSP2, a variant of #CSP where every variable appears even times, that the presumed universality holds for #CSP2. This becomes an important tool in the proof of the full dichotomy, which refutes this universality in general. The full dichotomy says that the new P-time algorithms and the strategy of holographic reductions to FKT together are universal for these locally defined counting problems. As a special case of our new planar tractable problems, counting perfect matchings (#PM) over k-uniform hypergraphs is P-time computable when the incidence graph is planar and k ≥ 5. The same problem is #P-hard when k = 3 or k = 4, also a consequence of the dichotomy. More generally, over hypergraphs with specified hyperedge sizes and the same planarity assumption, #PM is P-time computable if the greatest common divisor (gcd) of all hyperedge sizes is at least 5.