What Is Recursively Enumerable?

In mathematics, logic and computer science, a formal language is called recursively enumerable 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.

What does it mean to say a language L is recursively enumerable?

A language is recursively enumerable if there exists a Turing machine that accepts every string of the language, and does not accept strings that are not in the language. (Strings that are not in the language may be rejected or may cause the Turing machine to go into an infinite loop.)

What is the difference between recursive and recursively enumerable languages?

The main difference is that in recursively enumerable language the machine halts for input strings which are in language L. but for input strings which are not in L, it may halt or may not halt. When we come to recursive language it always halt whether it is accepted by the machine or not.