Natural Deduction Proofs Research Articles

A sequent calculus for a logic of contingencies

We introduce a sequent calculus that is sound and complete with respect to propositional contingencies, i.e., formulas which are neither provable nor refutable. Like many other sequent and natural deduction proof systems, this calculus possesses cut elimination and the subformula property and has a simple proof search mechanism.

A formal framework for specifying sequent calculus proof systems

Intuitionistic logic and intuitionistic type systems are commonly used as frameworks for the specification of natural deduction proof systems. In this paper we show how to use classical linear logic as a logical framework to specify sequent calculus proof systems and to establish some simple consequences of the specified sequent calculus proof systems. In particular, derivability of an inference rule from a set of inference rules can be decided by bounded (linear) logic programming search on the specified rules. We also present two simple and decidable conditions that guarantee that the cut rule and non-atomic initial rules can be eliminated.

Methods of Lemma Extraction in Natural Deduction Proofs

The existing examples of natural deduction proofs, either declarative or procedural, indicate that often the legibility of proof scripts is of secondary importance to the authors. As soon as the computer accepts the proof script, many authors do not work on improving the parts that could be shortened and do not avoid repetitions of technical sub-deductions, which often could be replaced by a single lemma. This article presents selected properties of reasoning passages that may be used to determine if a reasoning passage can be extracted from a proof script, transformed into a lemma and replaced by a reference to the newly created lemma. Additionally, we present methods for improving the legibility of the reasoning that remains after the extraction of the lemmas.

Proof Theory for Reasoning with Euler Diagrams: A Logic Translation and Normalization

Proof-theoretical notions and techniques, developed on the basis of sentential/symbolic representations of formal proofs, are applied to Euler diagrams. A translation of an Euler diagrammatic system into a natural deduction system is given, and the soundness and faithfulness of the translation are proved. Some consequences of the translation are discussed in view of the notion of free ride, which is mainly discussed in the literature of cognitive science as an account of inferential efficacy of diagrams. The translation enables us to formalize and analyze free ride in terms of proof theory. The notion of normal form of Euler diagrammatic proofs is investigated, and a normalization theorem is proved. Some consequences of the theorem are further discussed: in particular, an analysis of the structure of normal diagrammatic proofs; a diagrammatic counterpart of the usual subformula property; and a characterization of diagrammatic proofs compared with natural deduction proofs.

Minimal from classical proofs

Let A be a formula without implications, and Γ consist of formulas containing disjunction and falsity only negatively and implication only positively. Orevkov (1968) and Nadathur (2000) proved that classical derivability of A from Γ implies intuitionistic derivability, by a transformation of derivations in sequent calculi. We give a new proof of this result (for minimal rather than intuitionistic logic), where the input data are natural deduction proofs in long normal form (given as proof terms via the Curry–Howard correspondence) involving stability axioms for relations; the proof gives a quadratic algorithm to remove the stability axioms. This can be of interest for computational uses of classical proofs.

PROOF-THEORETIC SEMANTIC VALUES FOR LOGICAL OPERATORS

The paper proposes a semantic value for the logical constants (connectives and quantifiers) within the framework of proof-theoretic semantics, basic meaning on the introduction rules of a meaning conferring natural deduction proof system. The semantic value is defined based on Frege’s Context Principle, by taking “contributions” to sentential meanings as determined by the function-argument structure as induced by a type-logical grammar. In doing so, the paper proposes a novel proof-theoretic interpretation of the semantic types, traditionally interpreted in Henkin models. The compositionality of the resulting attribution of semantic values is discussed. Elsewhere, the same method was used for defining proof-theoretic meaning of subsentential phrases in a fragment of natural language. Doing the same for (the simpler and clearer case of) logic sheds more light on the proposal.

On the expressive power of schemes

We present a calculus, called the scheme-calculus, that permits to express natural deduction proofs in various theories. Unlike λ-calculus, the syntax of this calculus sticks closely to the syntax of proofs, in particular, no names are introduced for the hypotheses. We show that despite its non-determinism, some typed scheme-calculi have the same expressivity as the corresponding typed λ-calculi.

On the combination of logical and probabilistic models for information analysis

Formal logical tools are able to provide some amount of reasoning support for information analysis, but are unable to represent uncertainty. Bayesian network tools represent probabilistic and causal information, but in the worst case scale as poorly as some formal logical systems and require specialized expertise to use effectively. We describe a framework for systems that incorporate the advantages of both Bayesian and logical systems. We define a formalism for the conversion of automatically generated natural deduction proof trees into Bayesian networks. We then demonstrate that the merging of such networks with domain-specific causal models forms a consistent Bayesian network with correct values for the formulas derived in the proof. In particular, we show that hard evidential updates in which the premises of a proof are found to be true force the conclusions of the proof to be true with probability one, regardless of any dependencies and prior probability values assumed for the causal model. We provide several examples that demonstrate the generality of the natural deduction system by using inference schemes not supportable directly in Horn clause logic. We compare our approach to other ones, including some that use non-standard logics.

Proof-graphs: a Thorough Cycle Treatment, Normalization and Subformula Property

A normalization procedure is presented for a classical natural deduction (ND) proof system. This proof system, called N-Graphs, has a multiple conclusion proof structure, where cycles are allowed. With this, we have developed a thorough treatment of cycles, including cycles normalization via an algorithm. We also demonstrate the usefulness of the graphical framework of N-Graphs, where derivations are seen as digraphs. We use geometric perspective techniques to establish the normalization mechanism, thus giving a direct normalization proof. Moreover, the subformula and separation properties are determined.

Embedding Constructive K into Intuitionistic K

We investigate an embedding of CK natural deduction proofs into IK natural deduction proofs. CK and IK can both be regarded as intuitionistic analogs of the basic classical modal logic K. Since, in general, the proof theory of these logics is given by means of quite different techniques the embedding can be considered as an attempt to reconcile these two approaches. Further, we show that the embedding naturally extends to the case of CS4 and IS4, and propose a framework that allows one to obtain a modular approach for all the intermediate systems.

Visualizing Proof Search for Theorem Prover Development

We describe an interactive visualization tool for large natural deduction proof searches. The tool permits the display of a search as it progresses, as well as the proof tree itself. We discuss the feature set and architecture of the tool, including aspects of extensibility and the interface for interaction with other user-provided analysis and visualization code.

Data Compression for Proof Replay

We describe a compressing translation from SAT solver generated propositional resolution refutation proofs to classical natural deduction proofs. The resulting proof can usually be checked quicker than one that simply simulates the original resolution proof. We use this result in interactive theorem provers, to speed up reconstruction of SAT solver generated proofs. The translation is fast and scales up to large proofs with millions of inferences.

ATP-based Cross-Verification of Mizar Proofs: Method, Systems, and First Experiments

Mizar is a proof assistant used for formalization and mechanical verification of mathematics. The main use of Mizar is in the development of the Mizar Mathematical Library (MML), in which proofs are verified by the Mizar proof checker. The Mizar proof checker has a quite complex implementation, and also lacks the ability to print out detailed atomic proof steps in a format that is easy to verify with an independent proof-checking tool. This can raise concerns about the correctness of the MML. This paper describes a translation of Mizar natural-deduction proofs to the TPTP format used for recording derivations from first-order automated theorem proving systems, and verification of the resulting TPTP format derivations. The system was tested on two nontrivial sets of Mizar problems: the 252 “MPTP Challenge” problems, and 245 Mizar root theorems. The results of these tests are encouraging, and indicate that cross-verification of the whole MML is feasible.

A proof theory for machine code

This article develops a proof theory for low-level code languages. We first define a proof system, which we refer to as the sequential sequent calculus , and show that it enjoys the cut elimination property and that its expressive power is the same as that of the natural deduction proof system. We then establish the Curry-Howard isomorphism between this proof system and a low-level code language by showing the following properties: (1) the set of proofs and the set of typed codes is in one-to-one correspondence, (2) the operational semantics of the code language is directly derived from the cut elimination procedure of the proof system, and (3) compilation and decompilation algorithms between the code language and the typed lambda calculus are extracted from the proof transformations between the sequential sequent calculus and the natural deduction proof system. This logical framework serves as a basis for the development of type systems of various low-level code languages, type-preserving compilation, and static code analysis.

The AProS Project: Strategic Thinking & Computational Logic

The paper discusses tools for teaching logic used in Logic & Proofs, a web-based introduction to modern logic that has been taken by more than 1,300 students since the fall of 2003. The tools include a wide array of interactive learning environments or cognitive mini-tutors; most important among them is the Carnegie Proof Lab. The Proof Lab is a sophisticated interface for constructing natural deduction proofs and is central, as strategically guided discovery of proofs is the distinctive focus of the course. My discussion makes explicit the broader intellectual context, but also the pursuit of pedagogical goals and their experimental examination. The intellectual context includes i) the theoretical background for the proof search algorithm AProS and its use for a dynamic Proof Tutor, and ii) the programmatic expansion of the course to Computational Logic. (I recommend that the reader enter the virtual classroom of Logic & Proofs: the interactive components just cannot be properly reflected in a narrative. It is also very easy to download AProS and observe its ways of searching for proofs.)1

The Linear Logical Abstract Machine

We derive an abstract machine from the Curry-Howard correspondence with a sequent calculus presentation of Intuitionistic Propositional Linear Logic. The states of the register based abstract machine comprise a low-level code block, a register bank and a dump holding suspended procedure activations. Transformation of natural deduction proofs into our sequent calculus yields a type-preserving compilation function from the Linear Lambda Calculus to the abstract machine. We prove correctness of the abstract machine with respect to the standard call-by-value evaluation semantics of the Linear Lambda Calculus.

The geometry of non-distributive logics

AbstractIn this paper we introduce a new natural deduction system for the logic of lattices, and a number of extensions of lattice logic with different negation connectives. We provide the class of natural deduction proofs with both a standard inductive definition and a global graph-theoretical criterion for correctness, and we show how normalisation in this system corresponds to cut elimination in the sequent calculus for lattice logic. This natural deduction system is inspired both by Shoesmith and Smiley's multiple conclusion systems for classical logic and Girard's proofnets for linear logic.

Translation of resolution proofs into short first-order proofs without choice axioms

We present a way of transforming a resolution-style proof containing Skolemization into a natural deduction proof without Skolemization. The size of the proof increases only moderately (polynomially). This makes it possible to translate the output of a resolution theorem prover into a purely first-order proof that is moderate in size.

Simplifying Proofs in Fitch-Style Natural Deduction Systems

We present an algorithm for simplifying Fitch-style natural-deduction proofs in classical first-order logic. We formalize Fitch-style natural deduction as a denotational proof language, % MathType!...

Definitions by rewriting in the Calculus of Constructions

This paper presents general syntactic conditions ensuring the strong normalization and the logical consistency of the Calculus of Algebraic Constructions, an extension of the Calculus of Constructions with functions and predicates defined by higher-order rewrite rules. On the one hand, the Calculus of Constructions is a powerful type system in which one can formalize the propositions and natural deduction proofs of higher-order logic. On the other hand, rewriting is a simple and powerful computation paradigm. The combination of both allows, among other things, to develop formal proofs with a reduced size and more automation compared with more traditional proof assistants. The main novelty is to consider a general form of rewriting at the predicate-level which generalizes the strong elimination of the Calculus of Inductive Constructions.