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Program synthesis is the mechanised construction of software. One of the main difficulties is the efficient exploration of the very large solution space, and tools often require a user-provided syntactic restriction of the search space. While useful in general, such syntactic restrictions provide little help for the generation of programs that contain non-trivial constants, unless the user is able to provide the constants in advance. This is a fundamentally difficult task for state-of-the-art synthesisers. We propose a new approach to the synthesis of programs with non-trivial constants that combines the strengths of a counterexample-guided inductive synthesiser with those of a theory solver, exploring the solution space more efficiently without relying on user guidance. We call this approach CEGIS(T ), where T is a first-order theory. We present two exemplars, one based on Fourier-Motzkin (FM) variable elimination and one based on first-order satisfiability. We demonstrate the practical value of CEGIS(T ) by automatically synthesising programs for a set of intricate benchmarks. Additionally, we present a case study where we integrate CEGIS(T ) within the mature synthesiser CVC4 and show that CEGIS(T ) improves CVC4’s results. [ABSTRACT FROM AUTHOR]

This paper celebrates the scientific discoveries and the service to the automated reasoning community of Lawrence (Larry) T. Wos, who passed away in August 2020. The narrative covers Larry's most long-lasting ideas about inference rules and search strategies for theorem proving, his work on applications of theorem proving, and a collection of personal memories and anecdotes that let readers appreciate Larry's personality and enthusiasm for automated reasoning. [ABSTRACT FROM AUTHOR]

Algebraic datatypes, and among them lists and trees, have attracted a lot of interest in automated reasoning and Satisfiability Modulo Theories (SMT). Since its latest stable version, the SMT-LIB standard defines a theory of algebraic datatypes, which is currently supported by several mainstream SMT solvers. In this paper, we study this particular theory of datatypes and prove that it is strongly polite, showing how it can be combined with other arbitrary disjoint theories using polite combination. The combination method uses a new, simple, and natural notion of additivity that enables deducing strong politeness from (weak) politeness. [ABSTRACT FROM AUTHOR]

We present a number of alternative ways of handling transitive binary relations that commonly occur in first-order problems, in particular equivalence relations, total orders, and transitive relations in general. We show how such relations can be discovered syntactically in an input theory, and how they can be expressed in alternative ways. We experimentally evaluate different such ways on problems from the TPTP, using resolution-based reasoning tools as well as instance-based tools. Our conclusions are that (1) it is beneficial to consider different treatments of binary relations as a user, and that (2) reasoning tools could benefit from using a preprocessor or even built-in support for certain types of binary relations. [ABSTRACT FROM AUTHOR]

The AKS algorithm (by Agrawal, Kayal and Saxena) is a significant theoretical result, establishing "PRIMES in P" by a brilliant application of ideas from finite fields. This paper describes an implementation of the AKS algorithm in our theorem prover HOL4, together with a proof of its correctness and its computational complexity. The complexity analysis is based on a conservative computation model using a writer monad. The method is elementary, but enough to show that our implementation of the AKS algorithm takes a number of execution steps bounded by a polynomial function of the input size. This establishes formally that the AKS algorithm indeed shows "PRIMES is in P". [ABSTRACT FROM AUTHOR]

Model generation is a problem complementary to theorem proving and is important for fault analysis and debugging of formal specifications of security protocols, programs and terminological definitions, for example. This paper discusses several ways of enhancing the paradigm of bottom-up model generation, with the two main contributions being a new range-restriction transformation and generalized blocking techniques. The range-restriction transformation refines existing transformations to range-restricted clauses by carefully limiting the creation of domain terms. The blocking techniques are based on simple transformations of the input set together with standard equality reasoning and redundancy elimination techniques, and allow for finding small, finite models. All possible combinations of the introduced techniques and a classical range-restriction technique were tested on the clausal problems of the TPTP Version 6.0.0 with an implementation based on the SPASS theorem prover using a hyperresolution-like refinement. Unrestricted domain blocking gave best results for satisfiable problems, showing that it is an indispensable technique for bottom-up model generation methods, that yields good results in combination with both new and classical range-restricting transformations. Limiting the creation of terms during the inference process by using the new range-restricting transformation has paid off, especially when using it together with a shifting transformation. The experimental results also show that classical range restriction with unrestricted blocking provides a useful complementary method. Overall, the results show bottom-up model generation methods are good for disproving theorems and generating models for satisfiable problems, but less efficient for unsatisfiable problems. [ABSTRACT FROM AUTHOR]

This paper defines the (first-order) conflict resolution calculus: an extension of the resolution calculus inspired by techniques used in modern Sat-solvers. The resolution inference rule is restricted to (first-order) unit propagation and the calculus is extended with a mechanism for assuming decision literals and with a new inference rule for clause learning, which is a first-order generalization of the propositional conflict-driven clause learning procedure. The calculus is sound (because it can be simulated by natural deduction) and refutationally complete (because it can simulate resolution), and these facts are proven in detail here. [ABSTRACT FROM AUTHOR]

As a present to Mizar on its 40th anniversary, we develop an AI/ATP system that in 30 seconds of real time on a 14-CPU machine automatically proves 40 % of the theorems in the latest official version of the Mizar Mathematical Library ( MML). This is a considerable improvement over previous performance of large-theory AI/ATP methods measured on the whole MML. To achieve that, a large suite of AI/ATP methods is employed and further developed. We implement the most useful methods efficiently, to scale them to the 150000 formulas in MML. This reduces the training times over the corpus to 1-3 seconds, allowing a simple practical deployment of the methods in the online automated reasoning service for the Mizar users ( Miz $\mathbb {A}\mathbb {R}$). [ABSTRACT FROM AUTHOR]

The considerable mathematical knowledge encoded by the Flyspeck project is combined with external automated theorem provers (ATPs) and machine-learning premise selection methods trained on the Flyspeck proofs, producing an AI system capable of proving a wide range of mathematical conjectures automatically. The performance of this architecture is evaluated in a bootstrapping scenario emulating the development of Flyspeck from axioms to the last theorem, each time using only the previous theorems and proofs. It is shown that 39 % of the 14185 theorems could be proved in a push-button mode (without any high-level advice and user interaction) in 30 seconds of real time on a fourteen-CPU workstation. The necessary work involves: (i) an implementation of sound translations of the HOL Light logic to ATP formalisms: untyped first-order, polymorphic typed first-order, and typed higher-order, (ii) export of the dependency information from HOL Light and ATP proofs for the machine learners, and (iii) choice of suitable representations and methods for learning from previous proofs, and their integration as advisors with HOL Light. This work is described and discussed here, and an initial analysis of the body of proofs that were found fully automatically is provided. [ABSTRACT FROM AUTHOR]

In this paper we provide the first (as far as we know) direct calculus deciding satisfiability of formulae in negation normal form in the fragment of FHL (full hybrid logic with the binder, including the global and converse modalities), where no occurrence of a universal operator is in the scope of a binder. By means of a satisfiability preserving translation of formulae, the calculus can be turned into a satisfiability decision procedure for the fragment $\textsf{FHL}\setminus\Box \mathord\downarrow\Box$, i.e. formulae in negation normal form where no occurrence of the binder is both in the scope of and contains in its scope a universal operator. The calculus is based on tableaux and termination is achieved by means of a form of anywhere blocking with indirect blocking. Direct blocking is a relation between nodes in a tableau branch, holding whenever the respective labels (formulae) are equal up to (a proper form of) nominal renaming. Indirect blocking is based on a partial order on the nodes of a tableau branch, which arranges them into a tree-like structure. [ABSTRACT FROM AUTHOR]

This paper describes the Automated Reasoning for Mizar ( $\textsf{Miz}\mathbb{AR}$) service, which integrates several automated reasoning, artificial intelligence, and presentation tools with Mizar and its authoring environment. The service provides ATP assistance to Mizar authors in finding and explaining proofs, and offers generation of Mizar problems as challenges to ATP systems. The service is based on a sound translation from the Mizar language to that of first-order ATP systems, and relies on the recent progress in application of ATP systems in large theories containing tens of thousands of available facts. We present the main features of $\textsf{Miz}\mathbb{AR}$ services, followed by an account of initial experiments in finding proofs with the ATP assistance. Our initial experience indicates that the tool offers substantial help in exploring the Mizar library and in preparing new Mizar articles. [ABSTRACT FROM AUTHOR]

We present Infinox, an automated tool for analyzing first-order logic problems, aimed at showing finite unsatisfiability, i.e., the absence of models with finite domains. Finite satisfiability is not a decidable problem (only semi-decidable), which means that such a tool can never be complete. Nonetheless, our hope is that Infinox be a useful complement to finite model finders in practice. Infinox uses several different proof techniques for showing infinity of a set, each of which requires the identification of a function or a relation with particular properties. Infinox enumerates candidates to such functions and relations, and subsequently uses an automated theorem prover as a sub-procedure to try to prove the resulting proof obligations. We have evaluated Infinox on the relevant problems from the TPTP benchmark suite, and we are able to automatically show finite unsatisfiability for over 25% of these problems. [ABSTRACT FROM AUTHOR]

We present a case study illustrating how to exploit the expressive power of higher-order logic to complete a proof whose main lemma is already proved in a first-order theorem prover. Our proof exploits a link between the HOL4 and ACL2 proof systems to show correctness of a cone of influence reduction algorithm, implemented in ACL2, with respect to the classical semantics of linear temporal logic, formalized in HOL4. [ABSTRACT FROM AUTHOR]

Security protocols have been analysed focusing on a variety of properties to withstand the Dolev-Yao attacker. The Multi-Attacker treat model allows each protocol participant to behave maliciously intercepting and forging messages. Each principal may then behave as a Dolev-Yao attacker while neither colluding nor sharing knowledge with anyone else. This feature rules out the applicability of existing equivalence results in the Dolev-Yao model. The analysis of security protocols under the Multi-Attacker threat model brings forward yet more insights, such as retaliation attacks and anticipation attacks, which formalise currently realistic scenarios of principals competing each other for personal profit. They are variously demonstrated on a classical protocol, Needham-Schroeder's, and on a modern deployed protocol, Google's SAML-based single sign-on protocol. The general threat model for security protocols based on set-rewriting that was adopted in AVISPA (Armando et al. ) is extended to formalise the Multi-Attacker. The state-of-the-art model checker SATMC (Armando and Compagna, Int J Inf Secur 6(1):3-32, ) is then used to automatically validate the protocols under the new threats, so that retaliation and anticipation attacks can automatically be found. The tool support scales up to the Multi-Attacker threat model at a reasonable price both in terms of human interaction effort and of computational time. [ABSTRACT FROM AUTHOR]

Axiom pinpointing has been introduced in description logics (DL) to help the user understand the reasons why consequences hold by computing minimal subsets of the knowledge base that have the consequence in question (MinA). Most of the pinpointing algorithms described in the DL literature are obtained as extensions of tableau-based reasoning algorithms for computing consequences from DL knowledge bases. In this paper, we show that automata-based algorithms for reasoning in DLs and other logics can also be extended to pinpointing algorithms. The idea is that the tree automaton constructed by the automata-based approach can be transformed into a weighted tree automaton whose so-called behaviour yields a pinpointing formula, i.e., a monotone Boolean formula whose minimal valuations correspond to the MinAs. We also develop an approach for computing the behaviour of a given weighted tree automaton. We use the DL $\mathcal{SI}$ as well as Linear Temporal Logic (LTL) to illustrate our new pinpointing approach. [ABSTRACT FROM AUTHOR]

With the inclusion of an effective methodology, this article answers in detail a question that, for a quarter of a century, remained open despite intense study by various researchers. Is the formula XCB= e( x, e( e( e( x, y), e( z, y)), z)) a single axiom for the classical equivalential calculus when the rules of inference consist of detachment ( modus ponens) and substitution Where the function e represents equivalence, this calculus can be axiomatized quite naturally with the formulas e( x, x), e( e( x, y), e( y, x)), and e( e( x, y), e( e( y, z), e( x, z))), which correspond to reflexivity, symmetry, and transitivity, respectively. (We note that e( x, x) is dependent on the other two axioms.) Heretofore, thirteen shortest single axioms for classical equivalence of length eleven had been discovered, and XCB was the only remaining formula of that length whose status was undetermined. To show that XCB is indeed such a single axiom, we focus on the rule of condensed detachment, a rule that captures detachment together with an appropriately general, but restricted, form of substitution. The proof we present in this paper consists of twenty-five applications of condensed detachment, completing with the deduction of transitivity followed by a deduction of symmetry. We also discuss some factors that may explain in part why XCB resisted relinquishing its treasure for so long. Our approach relied on diverse strategies applied by the automated reasoning program OTTER. Thus ends the search for shortest single axioms for the equivalential calculus. [ABSTRACT FROM AUTHOR]

For almost a century, a treasure lay hidden in a library in Germany, hidden until a remarkable discovery was made. Indeed, for most of the twentieth century, all of science thought that Hilbert had posed twenty-three problems, and no others. In the mid-1990s, however, as a result of a thorough reading of Hilbert's files, a twenty-fourth problem was found (in a notebook, in file Cod. ms. D. Hilbert 600:3), a problem that might have a profound effect on research. This newly discovered problem focuses on the finding of simpler proofs and criteria for measuring simplicity. A proof may be simpler than previously known in one or more ways that include length, size (measured in terms of the total symbol count), and term structure. A simpler proof not only is more appealing aesthetically (and has fascinated masters of logic including C. A. Meredith, A. Prior, and I. Thomas) but is relevant to practical applications such as circuit design and program synthesis. This article presents Hilbert's twenty-fourth problem, discusses its relation to certain studies in automated reasoning, and offers researchers with varying interests the challenge of addressing this newly discovered problem. In particular, we include open questions to be attacked, questions that (in different ways and with diverse proof refinements as the focus) may prove of substantial interest to mathematicians, to logicians, and (perhaps in a slightly different manner) to those researchers primarily concerned with automated reasoning. [ABSTRACT FROM AUTHOR]

For close to a century, despite the efforts of fine minds that include Hilbert and Ackermann, Tarski and Bernays, Łukasiewicz, and Rose and Rosser, various proofs of a number of significant theorems have remained missing – at least not reported in the literature – amply demonstrating the depth of the corresponding problems. The types of such missing proofs are indeed diverse. For one example, a result may be guaranteed provable because of being valid, and yet no proof has been found. For a second example, a theorem may have been proved via metaargument, but the desired axiomatic proof based solely on the use of a given inference rule may have eluded the experts. For a third example, a theorem may have been announced by a master, but no proof was supplied. The finding of missing proofs of the cited types, as well as of other types, is the focus of this article. The means to finding such proofs rests with heavy use of McCune's automated reasoning program OTTER, reliance on a variety of powerful strategies this program offers, and employment of diverse methodologies. Here we present some of our successes and, because it may prove useful for circuit design and program synthesis as well as in the context of mathematics and logic, detail our approach to finding missing proofs. Well-defined and unmet challenges are included. [ABSTRACT FROM AUTHOR]

For more than three and one-half decades, beginning in the early 1960s, a heavy emphasis on proof finding has been a key component of the Argonne paradigm, whose use has directly led to significant advances in automated reasoning and important contributions to mathematics and logic. The theorems studied range from the trivial to the deep, even including some that corresponded to open questions. Often the paradigm asks for a theorem whose proof is in hand but that cannot be obtained in a fully automated manner by the program in use. The theorem whose hypothesis consists solely of the Meredith single axiom for two-valued sentential (or propositional) calculus and whose conclusion is the Łukasiewicz three-axiom system for that area of formal logic was just such a theorem. Featured in this article is the methodology that enabled the program OTTER to find the first fully automated proof of the cited theorem, a proof with the intriguing property that none of its steps contains a term of the form n( n( t)) for any term t. As evidence of the power of the new methodology, the article also discusses OTTER's success in obtaining the first known proof of a theorem concerning a single axiom of Łukasiewicz. [ABSTRACT FROM AUTHOR]

Since the first presentation of classical sentential logic as an axiomatic system by Frege in 1879, the study of a variety of sentential calculi has flourished. One major area of investigation, initiated by Łukasiewicz and his colleagues in the first half of the twentieth century and carried into the second half by Meredith, Thomas, Prior, et al., focuses on alternative axiom sets for such logics, and on formal proofs within them. This paper recalls a sampling of the results obtained heretofore, noting along the way how the papers in this special issue of the Journal of Automated Reasoning fit into that larger tradition of which they now form a part. It also suggests a number of further questions, open problems, and projects to which the methods developed in these papers seem ideally suited and might well be fruitfully applied. [ABSTRACT FROM AUTHOR]

In this special issue of the Journal of Automated Reasoning, this article sets the stage for the succeeding articles, all of which focus on finding proofs of theorems of formal logic and on the various methodologies that were employed. The proofs that are offered mark an important milestone for automated reasoning and for logic, for each of them is indeed new. One key question this article answers is why an automated reasoning program was able to find proofs that had eluded some of the finest mathematicians and logicians for many, many decades. [ABSTRACT FROM AUTHOR]

Experience has shown that large or multi-user interactive proof efforts can benefit significantly from structuring mechanisms, much like those available in many modern programming languages. Such a mechanism can allow some lemmas and definitions to be exported, and others not. In this paper we address two such structuring mechanisms for the ACL2 theorem prover: encapsulation and books. After presenting an introduction to ACL2, this paper justifies the implementation of ACL2's structuring mechanisms and, more generally, formulates and proves high-level correctness properties of ACL2. The issues in the present paper are relevant not only for ACL2 but also for other theorem-proving environments. [ABSTRACT FROM AUTHOR]

The specification of a function is often given by a logical formula, called a ∀∃-formula, of the following form: ∀x∃yΦ(x,y). More precisely, a specification is given in the context of a certain theory E and is stated by the judgment E ⊢ ∀x∃y Φ(x,y). In this paper, we consider the case in which E is an equational theory. It is divided into two parts. In the first part, we develop a theory for the automated proof of such judgments in the initial model of E . The validity in the initial model means that we consider not only equational theorems but also inductive ones. From our theory we deduce an automated method for the proof of a class of such judgments. In the second part, we present an automatedmethod for program synthesis. We show how the previous proof method can be used to generate a recursive program for a function f that satisfies a judgment E ⊢ ∀x Φ(x, f(x)). We illustrate our method with the automated synthesis of some recursive programs on domains such as integers and lists. Finally, we describe our system LEMMA, which is an implementation in Common Lisp of these new methods. [ABSTRACT FROM AUTHOR]

The research reported in this article was spawned by a colleague's request to find an elegant proof (of a theorem from Boolean algebra) to replace his proof consisting of 816 deduced steps. The request was met by finding a proof consisting of 100 deduced steps. The methodology used to obtain the far shorter proof is presented in detail through a sequence of experiments. Although clearly not an algorithm, the methodology is sufficiently general to enable its use for seeking elegant proofs regardless of the domain of study. In addition to (usually) being more elegant, shorter proofs often provide the needed path to constructing a more efficient circuit, a more effective algorithm, and the like. The methodology relies heavily on the assistance of McCune's automated reasoning program OTTER. Of the aspects of proof elegance, the main focus here is on proof length, with brief attention paid to the type of term present, the number of variables required, and the complexity of deduced steps. The methodology is iterative, relying heavily on the use of three strategies: the resonance strategy, the hot list strategy, and McCune's ratio strategy. These strategies, as well as other features on which the methodology relies, do exhibit tendencies that can be exploited in the search for shorter proofs and for other objectives. To provide some insight regarding the value of the methodology, I discuss its successful application to other problems from Boolean algebra and to problems from lattice theory. Research suggestions and challenges are also offered. [ABSTRACT FROM AUTHOR]

A logical entropy-based Informational Logic is presented which provides new tools for probabilistic automated reasoning and knowledge representation. Applications in automated theorem proving are examined, and a decision theory for probabilistic theorems is proposed. [ABSTRACT FROM AUTHOR]

This article discusses the two incarnations of Otter entered in the CADE-13 Automated Theorem Proving System Competition. Also presented are some historical background, a summary of applications that have led to new results in mathematics and logic, and a general discussion of Otter. [ABSTRACT FROM AUTHOR]