In computability theory, a set of natural numbers is called **computable**, **recursive**, or **decidable** if there is an algorithm which takes a number as input, terminates after a finite amount of time (possibly depending on the given number) and correctly decides whether the number belongs to the set or not.

A set which is not computable is called **noncomputable** or **undecidable**.

A more general class of sets than the computable ones consists of the computably enumerable (c.e.) sets, also called **semidecidable** sets. For these sets, it is only required that there is an algorithm that correctly decides when a number *is* in the set; the algorithm may give no answer (but not the wrong answer) for numbers not in the set.

A subset of the natural numbers is called **computable** if there exists a total computable function such that if and if . In other words, the set is computable if and only if the indicator function is computable.

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 subset of prime numbers is computable.
- A recursive language is a computable subset of a formal language.
- The set of Gödel numbers of arithmetic proofs described in Kurt Gödel's paper "On formally undecidable propositions of Principia Mathematica and related systems I" is computable; see Gödel's incompleteness theorems.

Non-examples:

Main article: List of undecidable problems |

- The set of Turing machines that halt is not computable.
- The isomorphism class of two finite simplicial complexes is not computable.
- The set of busy beaver champions is not computable.
- Hilbert's tenth problem is not computable.

If *A* is a computable set then the complement of *A* is a computable set. If *A* and *B* are computable sets then *A* ∩ *B*, *A* ∪ *B* and the image of *A* × *B* under the Cantor pairing function are computable sets.

*A* is a computable set if and only if *A* and the complement of *A* are both computably enumerable (c.e.). The preimage of a computable set under a total computable function is a computable set. The image of a computable set under a total computable bijection is computable. (In general, the image of a computable set under a computable function is c.e., but possibly not computable).

A is a computable set if and only if it is at level of the arithmetical hierarchy.

A is a computable set if and only if it is either the range of a nondecreasing total computable function, or the empty set. The image of a computable set under a nondecreasing total computable function is computable.