- Proof theory
Proof theory is a branch of
mathematical logicthat represents proofs as formal mathematical objects, facilitating their analysis by mathematical techniques. Proofs are typically presented as inductively-defined data structuressuch as plain lists, boxed lists, or trees, which are constructed according to the axioms and rules of inference of the logical system. As such, proof theory is syntactic in nature, in contrast to model theory, which is semantic in nature. Together with model theory, axiomatic set theory, and recursion theory, proof theory is one of the so-called "four pillars" of the foundations of mathematics.cite book
last = Wang
first = Hao
authorlink = Hao Wang (academic)
title = Popular Lectures on Mathematical Logic
publisher = Van Nostrand Reinhold Company
year = 1981
isbn = 0442231091
pages = 3–4] Proof theory can also be considered a branch of
philosophical logic, where the primary interest is in the idea of a proof-theoretic semantics, an idea which depends upon technical ideas in structural proof theoryto be feasible.
Although the formalisation of logic was much advanced by the work of such figures as
Gottlob Frege, Giuseppe Peano, Bertrand Russell, and Richard Dedekind, the story of modern proof theory is often seen as being established by David Hilbert, who initiated what is called Hilbert's programin the foundations of mathematics. Kurt Gödel's seminal work on proof theory first advanced, then refuted this program: his completeness theorem initially seemed to bode well for Hilbert's aim of reducing all mathematics to a finitist formal system; then his incompleteness theorems showed that this is unattainable. All of this work was carried out with the proof calculi called the Hilbert systems.
In parallel with the proof theoretic work of Gödel,
Gerhard Gentzenwas laying the foundations of what is now known as structural proof theory. In a few short years, Gentzen introduced the core formalisms of natural deduction(simultaneously with and independently of Jaskowski) and the sequent calculus, made fundamental advances in the formalisation of intuitionistic logic, introduced the important idea of analytic proof, and provided the first combinatorial proof of the consistency of Peano arithmetic.
Formal and informal proof
The "informal" proofs of everyday mathematical practice are unlike the "formal" proofs of proof theory. They are rather like high-level sketches that would allow an expert to reconstruct a formal proof at least in principle, given enough time and patience. For most mathematicians, writing a fully formal proof would have all the drawbacks of programming in
Formal proofs are constructed with the help of computers in
interactive theorem proving. Significantly, these proofs can be checked automatically, also by computer. (Checking formal proofs is usually trivial, whereas "finding" proofs ( automated theorem proving) is typically quite hard.) An informal proof in the mathematics literature, by contrast, requires weeks of peer reviewto be checked, and may still contain errors.
Kinds of proof calculi
The three most well-known styles of
*The Hilbert calculi
*The natural deduction calculi
*The sequent calculi
Each of these can give a complete and axiomatic formalization of propositional or
predicate logicof either the classical or intuitionistic flavour, almost any modal logic, and many substructural logics, such as relevance logicor linear logic. Indeed it is unusual to find a logic that resists being represented in one of these calculi.
As previously mentioned, the spur for the mathematical investigation of proofs in formal theories was
Hilbert's program. The central idea of this program was that if we could give finitary proofs of consistency for all the sophisticated formal theories needed by mathematicians, then we could ground these theories by means of a metamathematical argument, which shows that all of their purely universal assertions (more technically their provable sentences) are finitarily true; once so grounded we do not care about the non-finitary meaning of their existential theorems, regarding these as pseudo-meaningful stipulations of the existence of ideal entities.
The failure of the program was induced by
Kurt Gödel's incompleteness theorems, which showed that any ω-consistent theorythat is sufficiently strong to express certain simple arithmetic truths, cannot prove its own consistency, which on Gödel's formulation is a sentence.
Much investigation has been carried out on this topic since, which has in particular led to:
*Refinement of Gödel's result, particularly
J. Barkley Rosser's refinement, weakening the above requirement of ω-consistency to simple consistency;
*Axiomatisation of the core of Gödel's result in terms of a modal language,
*Transfinite iteration of theories, due to
Alan Turingand Solomon Feferman;
*The recent discovery of
self-verifying theories, systems strong enough to talk about themselves, but too weak to carry out the diagonal argument that is the key to Gödel's unprovability argument.
tructural proof theory
Structural proof theory is the subdiscipline of proof theory that studies proof calculi that support a notion of
analytic proof. The notion of analytic proof was introduced by Gentzen for the sequent calculus; there the analytic proofs are those that are cut-free. His natural deduction calculus also supports a notion of analytic proof, as shown by Dag Prawitz. The definition is slightly more complex: we say the analytic proofs are the normal forms, which are related to the notion of normal form in term rewriting. More exotic proof calculi such as Jean-Yves Girard's proof nets also support a notion of analytic proof.
Structural proof theory is connected to
type theoryby means of the Curry-Howard correspondence, which observes a structural analogy between the process of normalisation in the natural deduction calculus and beta reduction in the typed lambda calculus. This provides the foundation for the intuitionistic type theorydeveloped by Per Martin-Löf, and is often extended to a three way correspondence, the third leg of which are the cartesian closed categories.
linguistics, type-logical grammar, categorial grammarand Montague grammarapply formalisms based on structural proof theory to give a formal natural language semantics.
Tableau systems apply the central idea of analytic proof from structural proof theory to provide decision procedures and semi-decision procedures for a wide range of logics.
Ordinal analysis is a powerful technique for providing combinatorial consistency proofs for theories formalising arithmetic and analysis.
*J. Avigad, E.H. Reck, 2001 . [http://www.andrew.cmu.edu/user/avigad/Papers/infinite.pdf “Clarifying the nature of the infinite”: the development of metamathematics and proof theory] . Carnegie-Mellon Technical Report CMU-PHIL-120.
A. S. Troelstra, H. Schwichtenberg. "Basic Proof Theory" (Cambridge Tracts in Theoretical Computer Science). Cambridge University Press. ISBN 0-521-77911-1
*G. Gentzen. Investigations into logical deduction. In M. E. Szabo, editor, "Collected Papers of Gerhard Gentzen". North-Holland, 1969.
*L.Moreno-Armella & B.Sriraman (2005)."Structural Stability and Dynamic Geometry: Some Ideas on Situated Proof. International Reviews on Mathematical Education. Vol. 37, no.3, pp.130-139" [http://www.springerlink.com/content/n602313107541846/?p=74ab8879ce75445da488d5744cbc3818&pi=0]
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