In recursion theory (otherwise called the theory of computability) a set S of integers, or natural numbers, or literal strings, or tuples of any of the above, is recursive or computable or decidable if there is an algorithm that, when given a number or literal string or tuple (as the case may be) returns a correct yes-or-no answer to the question of whether the input number, string, or tuple is a member of the set S.

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