# Decidability

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**analysis in metalogic**- In metalogic: Discoveries about formal mathematical systems
…arrived at sharp concepts of decidability. In one sense, decidability is a property of sets (of sentences): that of being subject (or not) to mechanical methods by which to decide in a finite number of steps, for any closed sentence of a given formal system (e.g., of N), whether it…

Read More - In metalogic: The undecidability theorem and reduction classes
…this class of problems is undecidable is particularly suggestive. Once the concept of mechanical procedure was crystallized, it was relatively easy to find absolutely unsolvable problems—e.g., the halting problem, which asks for each Turing machine the question of whether it will ever stop, beginning with a blank tape. In other…

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- In metalogic: Discoveries about formal mathematical systems
**Hilbert’s theory of proofs**- In mathematics: Cantor
…that was consistent, complete, and decidable. By “consistent” Hilbert meant that it should be impossible to derive both a statement and its negation; by “complete,” that every properly written statement should be such that either it or its negation was derivable from the axioms; by “decidable,” that one should have…

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- In mathematics: Cantor
**problem in axiomatic method**- In foundations of mathematics: The axiomatic method
…a theorem (this is called decidability)? These questions were raised implicitly by David Hilbert (1862–1943) about 1900 and were resolved later in the negative, completeness by the Austrian-American logician Kurt Gödel (1906–78) and decidability by the American logician Alonzo Church (1903–95).

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- In foundations of mathematics: The axiomatic method
**set theory**- In set theory: Limitations of axiomatic set theory
…(such a sentence is called undecidable), (2) there is no algorithm (or iterative process) for deciding whether a sentence of ZFC is a theorem, and (3) these same statements hold for any consistent theory resulting from ZFC by the adjunction of further axioms or axiom schemas. Apparently ZFC can serve…

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- In set theory: Limitations of axiomatic set theory

### criteria in

**lower predicate calculus**- In formal logic: Validity in LPC
…that LPC is not a decidable system. This does not mean that it is never possible to prove that a given wff of LPC is valid—the validity of an unlimited number of such wffs can in fact be demonstrated—but it does mean that in the case of LPC, unlike that…

Read More - In formal logic: Axiomatization of LPC
Rules of uniform substitution for predicate calculi, though formulable, are mostly very complicated, and, to avoid the necessity for these rules, axioms for these systems are therefore usually given by axiom schemata in the sense explained earlier (

Read More*see above*Axiomatization of PC). Given the formation rules and definitions stated in… - In formal logic: Special systems of LPC
…system is that it is decidable. (The introduction of even a single dyadic predicate variable, however, would make the system undecidable, and, in fact, even the system that contains only a single dyadic predicate variable and no other predicate variables at all has been shown to be undecidable.)

Read More*b.*A still…

- In formal logic: Validity in LPC
**modal logic based on LPC**- In formal logic: Validity in modal logic
…is, like LPC itself, an undecidable system.

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- In formal logic: Validity in modal logic
**propositional calculus**- In formal logic: Validity in PC
…is said to be a decidable one. For other systems it can be proved that no decision procedure is possible; the decision problem for such a system is then said to be unsolvable, and the system is said to be an undecidable one.

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- In formal logic: Validity in PC