Input (grade 13): An example of a function with the real numbers as both its domain and codomain is the function f (x) = 2x, which assigns to every real number the real number with twice its value.
Output (grade 8): An example of an elementary function that acts on the real numbers as both the domain and the codomain could be f (x) = 2x. Every real number will be assigned a real number that is twice as big.