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Recursion theorem can refer to: The recursion theorem in set theory; Kleene's recursion theorem, also called the fixed point theorem, in computability theory; The master theorem (analysis of algorithms), about the complexity of divide-and-conquer algorithms
Gödel's proofs show that the set of logical consequences of an effective first-order theory is a computably enumerable set, and that if the theory is strong enough this set will be uncomputable. Similarly, Tarski's indefinability theorem can be interpreted both in terms of definability and in terms of computability.
A recursive step — a set of rules that reduces all successive cases toward the base case. For example, the following is a recursive definition of a person's ancestor. One's ancestor is either: One's parent (base case), or; One's parent's ancestor (recursive step). The Fibonacci sequence is another classic example of recursion: Fib(0) = 0 as ...
The second recursion theorem can be used to show that such equations define a computable function, where the notion of computability does not have to allow, prima facie, for recursive definitions (for example, it may be defined by μ-recursion, or by Turing machines).
As in the case of induction, we may treat different types of ordinals separately: another formulation of transfinite recursion is the following: Transfinite Recursion Theorem (version 2). Given a set g 1, and class functions G 2, G 3, there exists a unique function F: Ord → V such that F(0) = g 1, F(α + 1) = G 2 (F(α)), for all α ∈ Ord,
Examples: Every finite or cofinite subset of the natural numbers is computable. This includes these special cases: The empty set is computable. The entire set of natural numbers is computable. Each natural number (as defined in standard set theory) is computable; that is, the set of natural numbers less than a given natural number is computable.
The recursion theorem states that such a definition indeed defines a function that is unique. The proof uses mathematical induction. [1] An inductive definition of a set describes the elements in a set in terms of other elements in the set. For example, one definition of the set of natural numbers is:
The definition of a computably enumerable set as the domain of a partial function, rather than the range of a total computable function, is common in contemporary texts. This choice is motivated by the fact that in generalized recursion theories, such as α-recursion theory, the definition corresponding to domains has been found to be more ...