Recursive & Recursively Enumerable Languages Video Lecture | Question Bank for GATE Computer Science Engineering - Computer Science Engineering (CSE)

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1. What is the difference between a recursive language and a recursively enumerable language?
Ans. A recursive language is a language for which there exists an algorithm that can decide whether a given input belongs to the language or not. In other words, a recursive language is decidable. On the other hand, a recursively enumerable language is a language for which there exists an algorithm that can enumerate all the strings in the language, but it may not be able to decide whether a given string belongs to the language or not.
2. Can a recursively enumerable language be recursive as well?
Ans. Yes, a recursively enumerable language can also be recursive. If a language is recursively enumerable and there exists an algorithm that can decide whether a given input belongs to the language or not, then the language is both recursively enumerable and recursive. In other words, every recursive language is also recursively enumerable.
3. Are all recursive languages recursively enumerable?
Ans. Yes, all recursive languages are recursively enumerable. This is because if a language is recursive, then there exists an algorithm that can decide whether a given input belongs to the language or not. By modifying this algorithm slightly, we can also enumerate all the strings in the language. Therefore, every recursive language is also recursively enumerable.
4. Is it possible for a language to be neither recursive nor recursively enumerable?
Ans. Yes, it is possible for a language to be neither recursive nor recursively enumerable. Such languages are called non-recursive and non-recursively enumerable languages. These languages cannot be decided or enumerated by any algorithm. Examples of such languages include the language of all Turing machine descriptions that do not halt on empty input, and the language that contains all strings that encode a valid proof in a formal system.
5. Can a recursively enumerable language have an algorithm that decides it?
Ans. No, a recursively enumerable language cannot have an algorithm that decides it. A language can only be decided if it is recursive, but a recursively enumerable language may have an algorithm that can only enumerate its strings.
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