What is a function? A function from X to Y is a relation between X and Y
    that has the property that any two ordered pairs with the same x-value also
    have the same y-value. It is a correspondance that assigns to each number
    x in X exactly one number y in Y. Basically, X and Y are sets and there
    exists one number (x) in the set X that corresponds with one number (y)
    in the set Y.

    When dealing with functions, there are two basic types: the on-to function
    and the one-to-one function. A function is on-to if it it's range consists of all
    the values in the the set Y and it is one-to-one if there is one and only one
    x-value for each y-value in each of the sets. Below are some example of
    functions, see if you can determine whether they are on-to or one-to-one.

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