Recognizable language

In mathematics, logic and computer science, a formal language is called recursively enumerable (also recognizable, partially decidable, semidecidable, Turing-acceptable or Turing-recognizable) if it is a recursively enumerable subset in the set of all possible words over the alphabet of the language, i.e., if there exists a Turing machine which will enumerate all valid strings of the language.

Recursively enumerable languages are known as type-0 languages in the Chomsky hierarchy of formal languages. All regular, context-free, context-sensitive and recursive languages are recursively enumerable.

The class of all recursively enumerable languages is called RE.

There are three equivalent definitions of a recursively enumerable language:

All regular, context-free, context-sensitive and recursive languages are recursively enumerable.

Post's theorem shows that RE, together with its complement co-RE, correspond to the first level of the arithmetical hierarchy.

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