This paper is concerned with the complexity of proofs and of searching for proofs in two propositional proof system: Resolution and Polynomial Calculus (PC). For the former system we show that the recently proposed algorithm of [BW99] for searching for proofs cannot give better than weakly exponential performance. This is a consequence of showing optimality of their general relationship reffered to in [BW99] as size-width trade-off. We moreover obtain the optimality of the size-width trade-off for the widely used restrictions of resolution: Regular, Davis-Putnam, Negative, Positive and Linear. As for the second system, we show that the translation to polynomials of a CNF formula having short resolution proofs, cannot be refuted in PC with degree less than \Omega\Gammaan/ n). A consequence of this is that the simulation of resolution by PC of [CEI97] cannot be improved to better than quasipolynomial in the case we start with small resolution proofs. We conjecture that the simu...

We prove a new switching lemma that works for restrictions that set only a small fraction of the variables and is applicable to DNFs with small conjunctions. We use this to prove lower bounds for the Res(k) propositional proof system, an extension of resolution which works with k-DNFs instead of clauses. We also obtain an exponential separation between depth d circuits of k + 1.

HeerHugo is a propositional formula checker that determines whether a given formula is satisfiable or not. Its main ingredient is the branch/merge rule, that is inspired by an algorithm proposed by Stallmarck, which is protected by a software patent. The algorithm can be interpreted as a breadth first search algorithm. HeerHugo differs substantially from Stallmarck's algorithm, as it operates on formulas in conjunctive normal form and it is enhanced with many logical rules including unit resolution, 2-satisfiability tests and additional systematic reasoning techniques. In this paper, the main elements of the algorithm are discussed, and its remarkable effectiveness is illustrated with some examples and computational results.

An exponential lower bound for the size of tree-like Cutting Planes refutations of a certain family of CNF formulas with polynomial size resolution refutations is proved. This implies an exponential separation between the tree-like versions and the dag-like versions of resolution and Cutting Planes. In both cases only superpolynomial separations were known [29, 18, 8]. In order to prove these separations, the lower bounds on the depth of monotone circuits of Raz and McKenzie in [25] are extended to monotone real circuits. An exponential separation is also proved between tree-like resolution and several refinements of resolution: negative resolution and regular resolution. Actually this last separation also provides a separation between tree-like resolution and ordered resolution, thus the corresponding superpolynomial separation of [29] is extended. Finally, an exponential separation between ordered resolution and unrestricted resolution (also negative resolution) is proved. Only a superpolynomial separation between ordered and unrestricted resolution was previously known [13].

We prove an exponential lower bound for tree-like Cutting Planes refutations of a set of clauses which has polynomial size resolution refutations. This implies an exponential separation between tree-like and dag-like proofs for both CuttingPlanes and resolution; in both cases only superpolynomial separations were known before [30, 20, 10]. In order to prove this, we extend the lower bounds on the depth of monotone circuits of Raz and McKenzie [26] to monotone real circuits. In the case of resolution, we further improve this result by giving an exponential separation of tree-like resolution from (dag-like) regular resolution proofs. In fact, the refutation provided to give the upper bound respects the stronger restriction of being a Davis-Putnam resolution proof. This extends the corresponding superpolynomial separation of [30]. Finally, we prove an exponential separation between Davis-Putnam resolution and unrestricted resolution proofs; only a superpolynomial separation was previously...

Abstract. We study two resolution-like refutation systems for finitedomain constraint satisfaction problems, and the efficiency of these and of common CSP algorithms. By comparing the relative strength of these systems, we show that for instances with domain size d, backtracking with 2-way branching is super-polynomially more powerful than backtracking with d-way branching. We compare these systems with propositional resolution, and show that every family of CNF formulas which are hard for propositional resolution induces families of CSP instances that are hard for most of the standard CSP algorithms in the literature.

In this paper we introduce a definition of search trees for resolution based proof procedures. This definition describes more clearly the differences between the restrictions of resolution. Applying this concept to monotone restrictions of the resolution it is shown that the average proof length for propositional formulas is at most four times as large as for unrestricted resolution. The search trees used within this paper also allow the representation of space bounded resolution. 1 Introduction Many efforts have been made to classify proof procedures like resolution, restrictions of resolution and other systems like cutting plane systems, Davis--Putnam algorithms etc. with respect to the minimal proof length, e.g. see [6]. Also restrictions of the resolution proof procedure can be classified in this way [2], [3], [4], [5], [9], [10]. But these papers deal with worst--case complexities. In practice many restrictions show to be very efficient --- e.g. N--resolution --- whereas they are...

We consider a modification of the pigeonhole principle, MPHP , introduced by Goerdt in [7]. Using a technique of Razborov [9] and simplified by Impagliazzo, Pudl'ak and Sgall [8], we prove that any Polynomial Calculus refutation of a set of polynomials encoding the MPHP , requires degree \Omega\Gammagre n). We also prove that the this lower bound is tight, giving Polynomial Calculus refutations of MPHP of optimal degree.