Definition of Function "A function f from a set A to a set B is a relation that assigns to each element x in the set A exactly one element y in the set B. The set A is the domain (or set of inputs) of the function f, and the set B contains the range (or set of outputs)." See Set A and B below. A = {(1, 9), (2, 13), (3, 15), (4, 15), (5, 12), (6, 10)} I say Set A is a function because every x is assigned to exactly one y. Yes? B = {(1, 9), (2, 13), (3, 15), (2, 15), (5, 12), (6, 10)} I say Set B is not a function because x = 2 is assigned to two different values of y. Yes? Do not discuss one-to-one functions here. I am not there in the textbook.