EquiconsistencyIn mathematical logic, two theories are equiconsistent if the consistency of one theory implies the consistency of the other theory, and vice versa. In this case, they are, roughly speaking, "as consistent as each other". In general, it is not possible to prove the absolute consistency of a theory T. Instead we usually take a theory S, believed to be consistent, and try to prove the weaker statement that if S is consistent then T must also be consistent—if we can do this we say that T is consistent relative to S.
Intermediate logicIn mathematical logic, a superintuitionistic logic is a propositional logic extending intuitionistic logic. Classical logic is the strongest consistent superintuitionistic logic; thus, consistent superintuitionistic logics are called intermediate logics (the logics are intermediate between intuitionistic logic and classical logic). A superintuitionistic logic is a set L of propositional formulas in a countable set of variables pi satisfying the following properties: 1. all axioms of intuitionistic logic belong to L; 2.
Théorème d'élimination des coupuresEn logique mathématique, le théorème d'élimination des coupures (ou Hauptsatz de Gentzen) est le résultat central établissant l'importance du calcul des séquents. Il a été initialement prouvé par Gerhard Gentzen en 1934 dans son article historique « Recherches sur la déduction logique » pour les systèmes LJ et LK formalisant la logique intuitionniste et classique, respectivement.
Signature (logique)En calcul des prédicats et en algèbre universelle, une signature est une liste de symboles de constante, de fonction ou de relation, chacun ayant une arité. Dans certains formalismes, pour avoir moins de non-dit, la signature est une liste de couples (symbole, arité). La signature fournit les éléments primitifs pour la construction d'un langage du premier ordre sur cette signature. En calcul des prédicats à plusieurs types d'objets et en théorie des types, chaque symbole possède un type (l'arité n'est pas suffisante).
On Formally Undecidable Propositions of Principia Mathematica and Related Systems"Über formal unentscheidbare Sätze der Principia Mathematica und verwandter Systeme I" ("On Formally Undecidable Propositions of Principia Mathematica and Related Systems I") is a paper in mathematical logic by Kurt Gödel. Submitted November 17, 1930, it was originally published in German in the 1931 volume of Monatshefte für Mathematik. Several English translations have appeared in print, and the paper has been included in two collections of classic mathematical logic papers.
Théorie complèteEn logique mathématique, une théorie complète est une théorie qui est équivalente à un ensemble maximal cohérent de propositions ; ceci signifie qu'elle est cohérente et que toute extension propre ne l'est plus. Pour des théories logiques qui contiennent la logique propositionnelle classique, ceci équivaut à la condition que pour toute proposition φ du langage de la théorie, soit elle contient φ, soit elle contient sa négation ¬φ.
Self-verifying theoriesSelf-verifying theories are consistent first-order systems of arithmetic, much weaker than Peano arithmetic, that are capable of proving their own consistency. Dan Willard was the first to investigate their properties, and he has described a family of such systems. According to Gödel's incompleteness theorem, these systems cannot contain the theory of Peano arithmetic nor its weak fragment Robinson arithmetic; nonetheless, they can contain strong theorems.
Admissible ruleIn logic, a rule of inference is admissible in a formal system if the set of theorems of the system does not change when that rule is added to the existing rules of the system. In other words, every formula that can be derived using that rule is already derivable without that rule, so, in a sense, it is redundant. The concept of an admissible rule was introduced by Paul Lorenzen (1955). Admissibility has been systematically studied only in the case of structural (i.e.
