# Realizability

One of the forms of non-classical interpretation of logical and logico-mathematical languages. Various interpretations of realizability type are defined by the following scheme. For formulas of a logico-mathematical language one defines the relation "an object e realizes a closed formula F" , written "e r F" , for short. The definition is of an inductive nature: The relation $ e r F $
is first defined for atomic formulas $ F $,
and then for compound formulas under the assumption that the relation is already defined for the subformulas from which they are made up. A closed formula $ F $
is called realizable, or true for a given interpretation, if there is an object $ e $
such that $ e r F $.
A formula $ F $,
containing free variables $ x _ {1} \dots x _ {n} $,
is assumed to be realizable if the closed formula $ \forall x _ {1} {} \dots \forall x _ {n} F $
is realizable.

Such an interpretation, known as recursive realizability, was first proposed by S.C. Kleene (see [1], [2]), with the aim of giving an intuitionistic (constructive) semantics of the language of formal arithmetic in terms of recursive functions (cf. Recursive function). Other notions of realizability are modifications of recursive realizability.

The intuitive sense of the relation $ e r F $ is as follows: The object $ e $ encodes information from which one can derive the truth of the formula $ F $. For example, in recursive realizability, the natural number 0 realizes an elementary formula of the form $ s = t $ if and only if this formula is true (i.e. the values of the terms $ s $ and $ t $ coincide); if a number $ e $ realizes the disjunction $ A \lor B $, then $ e $ encodes which of the formulas $ A $ or $ B $ is realizable, and produces a number realizing this formula. If a number $ e $ realizes the formula $ \forall x A( x) $, then $ e $ encodes an algorithm which, for any natural number $ n $, produces a realization of the formula $ A ( n) $. As realizers, i.e. objects which realize formulas, one can almost always take natural numbers. However, in the intuitionistic interpretation of the language of mathematical analysis, other objects can be used as realizers, such as unary number-theoretical functions (see, for example, [3]).

For the formulas of a logical language, such as propositional or predicate formulas, realizability is usually defined using the concept of realizability for some logico-mathematical language $ \Omega $. A logical formula $ \mathfrak A $ is said to be realizable if every formula in the language $ \Omega $ that can be obtained from $ \mathfrak A $ by substituting formulas of $ \Omega $ for the predicate variables is realizable.

Realizability interpretations have found wide application in the study of non-classical, mostly intuitionistic and constructive, logical and logico-mathematical theories. For a description of various notions of realizability and their applications in proof theory for investigations in intuitionistic theories see [3], [4].

#### References

[1] | S.C. Kleene, "On the interpretation of intuitionistic number theory" J. Symbolic Logic , 10 (1945) pp. 109–124 |

[2] | S.C. Kleene, "Introduction to metamathematics" , North-Holland (1951) |

[3] | S.C. Kleene, R.E. Vesley, "The foundations of intuitionistic mathematics: especially in relation to recursive functions" , North-Holland (1965) |

[4] | A.G. Dragalin, "Mathematical intuitionism. Introduction to proof theory" , Amer. Math. Soc. (1988) (Translated from Russian) |

#### Comments

J.M.E. Hyland [a1] has given a "model-theoretic" version of realizability, by showing that it is the internal logic of a certain topos, the effective topos or Hyland realizability topos.

#### References

[a1] | J.M.E. Hyland, "The effective topos" A.S. Troelstra (ed.) D. van Dalen (ed.) , The L.E.J. Brouwer Centenary Symposium , North-Holland (1982) pp. 165–216 |

**How to Cite This Entry:**

Realizability.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Realizability&oldid=48450