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13:00-15:00 Session 6A: SAT, SMT and QBF
The Resolution of Keller's Conjecture

ABSTRACT. We consider three graphs, G_{7,3}, G_{7,4}, and G_{7,6}, related to Keller's conjecture in dimension 7. The conjecture is false for this dimension if and only if at least one of the graphs contains a clique of size 2^7 = 128. We present an automated method to solve this conjecture by encoding the existence of such a clique as a propositional formula. We apply satisfiability solving combined with symmetry-breaking techniques to determine that no such clique exists. This result implies that every unit cube tiling of R^7 contains a facesharing pair of cubes. Since a faceshare-free unit cube tiling of R^8$exists (which we also verify), this completely resolves Keller's conjecture.

How QBF Expansion Makes Strategy Extraction Hard

ABSTRACT. In this paper we show that the QBF proof checking format QRAT (Quantified Resolution Asymmetric Tautologies) by Heule, Biere and Seidl cannot have polynomial time strategy extraction unless P=PSPACE. In our proof, the crucial property that makes strategy extraction PSPACE-hard for this proof format is universal expansion, even expansion on a single variable.

While expansion reasoning used in other QBF calculi can admit polynomial time strategy extraction, we find this is conditional on a property studied in proof complexity theory. We show that strategy extraction on expansion based systems can only happen when the underlying propositional calculus has the property of feasible interpolation.

Covered Clauses Are Not Propagation Redundant

ABSTRACT. Propositional proof systems based on recently-developed redundancy properties admit short refutations for many formulas traditionally considered hard. Redundancy properties are also used by procedures which simplify formulas in conjunctive normal form by removing redundant clauses. Revisiting the covered clause elimination procedure, we prove the correctness of an explicit algorithm for identifying covered clauses, as it has previously only been implicitly described. While other elimination procedures produce redundancy witnesses for compactly reconstructing solutions to the original formula, we prove that witnesses for covered clauses are hard to compute. Further, we show that not all covered clauses are propagation redundant, the most general, polynomially-verifiable standard redundancy property. Finally, we close a gap in the literature by demonstrating the complexity of clause redundancy itself.

Solving bit-vectors with MCSAT: explanations from bits and pieces

ABSTRACT. We develop a treatment of the theory of fixed-sized bit-vectors in the MCSAT framework. MCSAT is an alternative to CDCL(T) for SMT solving and can be seen as an extension of CDCL to other domains than the Booleans. To apply MCSAT to bit-vectors, we use Binary Decision Diagrams to record and update the set of feasible values for a bit-vector variable and we develop specialized conflict-explanation mechanisms. We present two methods based on interpolation that generate word-level explanations. These methods are applicable when the conflict is restricted to two fragments of bit-vector arithmetic. We use aggressive normalization to shoehorn as many conflicts as possible into these fragments. When that fails, we resort to bit-blasting to generate explanations. The approach is implemented in the Yices 2 SMT solver. We present experimental results.

13:00-15:00 Session 6B: Isabelle
Reasoning about Algebraic Structures with Implicit Carriers in Isabelle/HOL

ABSTRACT. We prove Chen and Grätzer's construction theorem for Stone algebras in Isabelle/HOL. The development requires extensive reasoning about algebraic structures in addition to reasoning in algebraic structures. We present an approach for this using classes and locales with implicit carriers. This involves using function liftings to implement some aspects of dependent types and using embeddings of algebras to inherit theorems. We also formalise a theory of filters based on partial orders.

Algebraically Closed Fields in Isabelle/HOL

ABSTRACT. That every field admits an algebraically closed extension is a fundamental mathematical theorem. Despite its central importance, this result has never before been formalised in a proof assistant. We fill this gap by documenting its formalisation in Isabelle/HOL, describing the difficulties that impeded this development and their solutions.

Verifying Faradžev-Read type Isomorph-Free Exhaustive Generation

ABSTRACT. Many applications require generating catalogues of combinatorial objects with certain properties, that do not contain isomorphs. Several efficient general schemes for this problem exist. One is described independently by I. A. Faradžev and R. C. Read and has since been applied to catalogue many different combinatorial structures. We present an Isabelle/HOL verification of this abstract scheme. To show its practicality, we instantiate it on two concrete problems: enumerating digraphs and enumerating union-closed families of sets. In the second example abstract algorithm specification is refined to an implementation that can quite efficiently enumerate all canonical union-closed families over a six element universe (there is more than 100 million such families).

Formalization of Forcing in Isabelle/ZF

ABSTRACT. We formalize the theory of forcing in the set theory framework of Isabelle/ZF. Under the assumption of the existence of a countable transitive model of ZFC, we construct a proper generic extension and show that the latter also satisfies ZFC. In doing so, we remodularized Paulson's ZF-Constructibility library.

15:15-16:15 Session 7: Invited Talk: E. Pimentel
Focusing, axioms and synthetic inference rules (extended abstract)
16:30-18:30 Session 8A: Non Classical Logics
Soft subexponentials and multiplexing

ABSTRACT. Linear logic and its refinements have been used as a specification language for a number of deductive systems. This has been accomplished by carefully studying the structural restrictions of linear logic modalities. Examples of such refinements include subexponentials, light linear logic, and soft linear logic. We bring together these refinements of linear logic in a non-commutative setting. We introduce a non-commutative substructural system with subexponential modalities controlled by a minimalistic set of rules. Namely, we disallow the contraction and weakening rules for the exponential modality and introduce two primitive subexponentials. One of the subexponentials allows the multiplexing rule in the style of soft linear logic and light linear logic. The second subexponential provides the exchange rule. For this system, we construct a sequent calculus, establish cut elimination, and also provide a complete focused proof system. We illustrate the expressive power of this system by simulating Turing computations and categorial grammar parsing for compound sentences. Using the former, we prove an undecidability result. The new system employs Lambek's non-emptiness restriction, which is incompatible with the standard (sub)exponential setting. Lambek's restriction is crucial for applications in linguistics: without this restriction, categorial grammars incorrectly mark some ungrammatical phrases as being correct.

Description Logics with Concrete Domains and General Concept Inclusions Revisited

ABSTRACT. Concrete domains have been introduced in the area of Description Logic to enable reference to concrete objects (such as numbers) and predefined predicates on these objects (such as numerical comparisons) when defining concepts. Unfortunately, in the presence of general concept inclusions (GCIs), which are supported by all modern DL systems, adding concrete domains may easily lead to undecidability. One contribution of this paper is to strengthen the existing undecidability results further by showing that concrete domains even weaker than the ones considered in the previous proofs may cause undecidability. To regain decidability in the presence of GCIs, quite strong restrictions, in sum called omega-admissiblity, need to be imposed on the concrete domain. On the one hand, we generalize the notion of omega-admissiblity from concrete domains with only binary predicates to concrete domains with predicates of arbitrary arity. On the other hand, we relate omega-admissiblity to well-known notions from model theory. In particular, we show that finitely bounded, homogeneous structures yield omega-admissible concrete domains. This allows us to show omega-admissibility of concrete domains using existing results from model theory.

Constructive Hybrid Games

ABSTRACT. Hybrid games are models which combine discrete, continuous, and adversarial dynamics. Game logic enables proving existence of (perhaps noncomputable) winning strategies. We introduce constructive differential game logic (CdGL) for hybrid games, where proofs that a player can win the game correspond to computable winning strategies. This is the logical foundation for synthesis of correct control and monitoring code for safety-critical cyber-physical systems. Our contributions include novel static and dynamic semantics as well as results such as soundness and consistency.

NP Reasoning in the Monotone mu-Calculus

ABSTRACT. Satisfiability checking for monotone modal logic is known to be (only) NP-complete. We show that this remains true when the logic is extended with alternation-free fixpoint operators as well as the universal modality; the resulting logic – the alternation-free monotone µ-calculus with the universal modality – contains both concurrent propositional dynamic logic (CPDL) and the alternation-free fragment of game logic as fragments. We obtain our result from a characterization of satisfiability by means of Büchi games with polynomially many Eloise nodes.

16:30-18:00 Session 8B: Reasoning Systems, Tactics and Tools
A Programmer’s Text Editor for a Logical Theory: The SUMOjEdit Editor (system description)

ABSTRACT. SUMOjEdit is a programmer’s text editor for the SUO-KIF language and SUMO theory. Modern procedural programming is done in a text editor with tool support. Development of ontologies and taxonomies has often been done in graphical editors, leading many developers to employ only logics of very limited expressiveness that can be manipulated visually. Developers in the theorem proving community typically work in text editors but without the same degree of tool support that most programmers rely on. Beginners working with SUMO make some very predictable errors in syntax, logical formulation, and use of the library of theories. Many of these errors can be flagged during editing, resulting in reduced time to become a productive developer. An editor designed for working with SUMO has the potential to aid beginners and experienced SUMO developers.

Sequoia: a playground for logicians (system description)

ABSTRACT. Sequent calculus is a pervasive technique for studying logics and their properties due to the regularity of rules, proofs, and meta-property proofs across logics. However, even simple proofs can be large, and writing them by hand is often messy. Moreover, the combinatorial nature of the calculus makes it easy for humans to make mistakes or miss cases. Sequoia aims to alleviate these problems.

Sequoia is a web-based application for specifying sequent calculi and performing basic reasoning about them. The goal is to be a user-friendly program, where logicians can specify and "play" with their calculi. For that purpose, we provide an intuitive interface where inference rules can be input in LaTeX and are immediately rendered with the corresponding symbols. Users can then build proof trees in a streamlined and minimal-effort way, in whichever calculus they defined. In addition to that, we provide checks for some of the most important meta-theoretical properties, such as weakening admissibility and identity expansion, given that they proceed by the usual structural induction. In this sense, the logician is only left with the tricky and most interesting cases of each analysis.

HYPNO: Theorem Proving with Hypersequent Calculi for Non-Normal Modal Logics (system description)

ABSTRACT. We present HYPNO (HYpersequent Prover for NOn-normal modal logics), a Prolog-based theorem prover and countermodel generator for non-normal modal logics. HYPNO implements some hypersequent calculi recently introduced for the basic system E and its extensions with axioms M, N, and C. It is inspired by the methodology of LeanTAP, so that it does not make use of any ad-hoc control mechanism. Given a formula, HYPNO provides either a proof in the calculus or a countermodel, directly built from an open saturated branch. Preliminary experimental results show that the performances of HYPNO are very promising with respect to other theorem provers for the same class of logics.

Moin: A Nested Sequent Theorem Prover for Intuitionistic Modal Logics (system description)

ABSTRACT. We present a simple Prolog prover for intuitionistic modal logics based on nested sequent proof systems. We have implemented single-conclusion (Genzten-style) nested sequent systems and multi-conclusion nested sequent systems (Maehara-style) for all logics in the intuitionistic modal IS5-cube. While the single-conclusion system are better investigated and have an internal cut-elimination, the multi-conclusion systems can provide a counter model in case the proof search fails. To our knowledge this is the first automated theorem prover for intuitionistic modal logics. For wider usability of our system, we also implemented all classical normal modal logics in the S5-cube.

A Lean tactic for normalising ring expressions with exponents (short paper)

ABSTRACT. This paper describes the design of the normalising tactic ring_exp for the Lean prover. This tactic improves on existing tactics by extending commutative rings with a binary exponent operator. An inductive family of types represents the normal form, enforcing various invariants. The design can also be extended with more operators.

Logic-Independent Proof Search in Logical Frameworks (short paper)

ABSTRACT. Logical frameworks enable prototyping and analyzing logical systems as well as developing logic-independent implementations. An important question is to what extent the latter can include a logic-independent automated theorem prover. On the one hand, many theorem proving innovations can be generalized to much larger classes of logics than they were originally developed for. On the other hand, most practically competitive provers critically use logic-specific optimizations. Indeed, logical framework-induced fully logic-independent proof support is generally limited to proof checking and simple search.

In order to investigate this question more deeply, we build a suite of highly modular definitions of a variety of logics in a logical framework and generate Prolog-based theorem provers for each of them. We emphasize generality of the method and try to avoid taking any logic-specific knowledge into account. Our generated provers support natural deduction proof search, including back-chaining, as well as tableau provers and model generators. Even though these efforts have only just started, we are already able to use the latter in a system for natural language understanding that combines grammatical with semantic processing.

18:30-20:00 Session 9: Business Meetings (IJCAR, FROCOS, TABLEAUX, CADE)

Report of IJCAR 2020 PC co-chairs (N. Peltier)

Best Paper Award (V. Sofronie-Stokkermans)

Report of IJCAR 2020 Conference Chair (K. Chaudhuri)

Information regarding CADE 2021, FroCoS 2021, Tableaux 2021, and IJCAR 2022

Chairs: Franz Baader, Jens Otten, Christoph Weidenbach