In computer science and engineering, a system acts as a computable function.
The class of computable functions that return 0 for every input, and its complement.
The class of computable functions that are constant, and its complement.
Let be a set of computable functions such that .
This term has since come to be identified with the computable functions.
The greatest common divisor of two numbers is a computable function.
R is equal to the set of all total computable functions.
Each computable function has an infinite number of different program representations in a given programming language.
To complete the proof, let be any total computable function, and construct as above.
Although Kleene did not give examples of "computable functions" others have.