Recursively Enumerable Language - Definitions

Definitions

There exist three equivalent major definitions for the concept of a recursively enumerable language.

  1. A recursively enumerable language is a recursively enumerable subset in the set of all possible words over the alphabet of the language.
  2. A recursively enumerable language is a formal language for which there exists a Turing machine (or other computable function) which will enumerate all valid strings of the language. Note that, if the language is infinite, the enumerating algorithm provided can be chosen so that it avoids repetitions, since we can test whether the string produced for number n is "already" produced for a number which is less than n. If it already is produced, use the output for input n+1 instead (recursively), but again, test whether it is "new".
  3. A recursively enumerable language is a formal language for which there exists a Turing machine (or other computable function) that will halt and accept when presented with any string in the language as input but may either halt and reject or loop forever when presented with a string not in the language. Contrast this to recursive languages, which require that the Turing machine halts in all cases.

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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