We consider the task of weighted first-order model counting (WFOMC), a fundamental problem of probabilistic inference in statistical relational learning. The goal of WFOMC is to compute the weighted sum of models of a given first-order logic sentence over a finite domain, where each model is assigned a weight by a pair of weighting functions. Past work has shown that WFOMC can be solved in polynomial time in the domain size if the sentence is in the two-variable fragment of first-order logic (FO2). This result is later extended to the case where the sentence is in FO2with the linear order axiom, which requires a binary predicate in the sentence to introduce a linear ordering of the domain elements. However, despite its polynomial theoretical complexity, the existing domain-liftable algorithm for WFOMC with the linear order often suffers from inefficiencies when applied to real-world problems. This paper introduces a novel domain-lifted algorithm for WFOMC with the linear order axiom. Compared to the existing approach, our proposed algorithm exploits the inherent symmetries within first-order logic sentences and weighting functions to minimize redundant computations. Experimental results verify the efficiency of our approach, demonstrating a significant speedup over the existing approach.

We study the time complexity of weighted first-order model counting (WFOMC) over the logical language with two variables and counting quantifiers. The problem is known to be solvable in time polynomial in the domain size. However, the degree of the polynomial, which turns out to be relatively high for most practical applications, has never been properly addressed. First, we formulate a time complexity bound for the existing techniques for solving WFOMC with counting quantifiers. The bound is already known to be a polynomial with its degree depending on the number of cells of the input formula. We observe that the number of cells depends, in turn, exponentially on the parameters of the counting quantifiers appearing in the formula. Second, we propose a new approach to dealing with counting quantifiers, reducing the exponential dependency to a quadratic one, therefore obtaining a tighter upper bound. It remains an open question whether the dependency of the polynomial degree on the counting quantifiers can be reduced further, thus making our new bound a bound to beat.

We consider the task of weighted first-order model counting (WFOMC) used for probabilistic inference in the area of statistical relational learning. Given a formula φ, domain size n and a pair of weight functions, what is the weighted sum of all models of φ over a domain of size n? It was shown that computing WFOMC of any logical sentence with at most two logical variables can be done in time polynomial in n. However, it was also shown that the task is #P1-complete once we add the third variable, which inspired the search for extensions of the two-variable fragment that would still permit a running time polynomial in n. One of such extension is the two-variable fragment with counting quantifiers. In this paper, we prove that adding a linear order axiom (which forces one of the predicates in φ to introduce a linear ordering of the domain elements in each model of φ) on top of the counting quantifiers still permits a computation time polynomial in the domain size. We present a new dynamic programming-based algorithm which can compute WFOMC with linear order in time polynomial in n, thus proving our primary claim.

We study the problem of generating interesting integer sequences with a combinatorial interpretation. For this we introduce a two-step approach. In the first step, we generate first-order logic sentences which define some combinatorial objects, e.g., undirected graphs, permutations, matchings etc. In the second step, we use algorithms for lifted first-order model counting to generate integer sequences that count the objects encoded by the first-order logic formulas generated in the first step. For instance, if the first-order sentence defines permutations then the generated integer sequence is the sequence of factorial numbers n!. We demonstrate that our approach is able to generate interesting new sequences by showing that a non-negligible fraction of the automatically generated sequences can actually be found in the Online Encyclopaedia of Integer Sequences (OEIS) while generating many other similar sequences which are not present in OEIS and which are potentially interesting. A key technical contribution of our work is the method for generation of first-order logic sentences which is able to drastically prune the space of sentences by discarding large fraction of sentences which would lead to redundant integer sequences.