The main form of computability studied in recursion theory was introduced by Turing (1936). A set of natural numbers is said to be a "computable set" (also called a "decidable", "recursive", or "Turing computable" set) if there is a Turing machine that, given a number "n", halts with output 1 if "n" is in the set and halts with output 0 if "n" is not in the set. A function "f" from the natural numbers to themselves is a "recursive" or "(Turing) computable function" if there is a Turing machine that, on input "n", halts and returns output "f"("n"). The use of Turing machines here is not necessary; there are many other models of computation that have the same computing power as Turing machines; for example the μ-recursive functions obtained from primitive recursion and the μ operator.