Onto Function

A function is an onto function if every element or y-value in the range comes from at least one element or x-value in the domain. In addition, it is a MUST that all elements of the range and codomain are the same.

Therefore, it is possible for two or more elements in the domain to be paired or associated with the same y-value in the range/codomain. No elements in the range or codomain must be left out.