when is a function said to be onto
Surjective function In mathematics, a function f from a set X to a set Y is surjective (or onto), or a surjection, if for every element y in the codomain Y of f there is at least one element x in the domain X of f such that f(x) = y. It is not required that x is unique; the function f may map one or more elements of X to the same element of Y.
Abscissa and ordinate Usually these are the horizontal and vertical coordinates of a point in a two-dimensional rectangular Cartesian coordinate system. The terms can also refer to the horizontal and vertical axes respectively (typically x-axis and y–axis) of a two-dimensional graph. An ordered pair consists of two terms—the abscissa (horizontal, usually x) and the ordinate (vertical, usually y)—which define the location of a point in two-dimensional rectangular space.