1.2 Functions and Their Properties

A function is actually a simple concept; if it were not, history would have replaced it with a simpler one by now! Here is the definition:

Definition: Function, Domain, and Range
A function from a set D to a set R is a rule that assigns to every element in D a unique element in R. The set D of all input values is the ______of the function, and the set R of all output values is the ______of the function.

There are many ways to look at functions. At some point in your math career, your teacher may have talked to you about a “function machine”, in which values of the domain (x) are fed into the machine (the function f) to produce range values (y). A very concise way to state this is using the function notation: y = f(x), read as “y equals f of x”.

A function can also be viewed as a mapping of the elements of the domain onto the elements of the range. We’ll sketch two figures below: one that is a function and one that is not.

FUNCTION MAPPING:NOT A FUNCTION MAPPING:

The uniqueness of the range value is very important. Knowing that f(2)= 8 tells us something about f, and that understanding would be contradicted if we were to discover later that f(2) = 4. Why will you never see a function defined by an ambiguous formula like f(x) = 2?

Another useful way to look at functions is graphically. The graph of the function y = f(x)is the set of all points (x, f(x)), with x in the domain of f. We match domain values along the x-axis to get the ordered pairs that yield the graph of y = f(x). Draw a graph that is a function and a graph that is not a function

FUNCTION SKETCH:NOT A FUNCTION SKETCH:

One useful way to determine whether or not a relation is a function graphically is to use the vertical line test.

Vertical Line Test
A graph (set of points (x, y)) in the xy-plane defines y as a function of x if and only if no vertical line intersects the graph in ______.

Function or not?

Now that we know what a function is and what one does and does not look like we need to think about other terminology that can be used to describe a function. While some of the terms that follow may be familiar to you, we will take time to carefully review each one by giving a formal definition and then reviewing some illustrative examples of that concept to help cement your understanding.

Domain and Range
Domain / The inputs where the function is defined. Exclude x-values that divide by zero or take an even root of a negative number.
Range / The output values of the function.

Generally, we will define functions algebraically and we will assume that the domain of the function defined by the algebraic expression is the same as the domain of the algebraic expression itself. That is, we will use the implied domain. Here is an example that should help illustrate this point: The volume formula for a circle is: and the implied domain for this formula would be all real numbers; however, the volume function is not defined for negative radius values. This means that if our intent were to study the volume function, we would need to restrict the radius to values of 0 or higher (we call this the relevant domain).

Example 1: Finding the Domain and Range of a Function

Find the domain and range of each of these functions:

(a)

(b)

(c)

While a formal definition of continuity involving limit notation is vital to the study of Calculus, an informal understanding will be sufficient at this time. Basically, the mathematical definition of continuity captures two ideas: (1) the values of afunctionf(x) at points near ‘a’ are good predictors of the value of f at ‘a’ and (2) the graph of f is a connected curve with no jumps, gaps, or holes. If either of these conditions is violated, the function is considered to be discontinuous.

There are two types of discontinuities to consider – removable and non-removable.

A removable discontinuity is one at which the limit of the function exists, but it does not equal the value of the function at that point. Informally, we think of this as a discontinuity that can be “repaired” – it is a point at which a graph is not connected but can be made connected by filling in a single point.

A non-removable (or essential) discontinuity is one at which the limit of the function does not exist. Informally, it is a place where a graph is not connected and cannot be made connected simply by filling in a single point. Step (or jump) discontinuities and vertical asymptotes (which create an infinite discontinuity) are two types of non-removable discontinuities.

Let’s illustrate these ideas with a few graphs:

Continuity
Continuous / Removable
(Hole, f(a) exists) / Removable
(Hole, f(a) does not exist)
Non-removable (Step or Jump) / Non-removable
(Infinite, vertical asymptote on one side) / Non-removable
(Infinite, vertical asymptote on both sides)

Example 2: Determining Continuity

Determine if each function is continuous. If the function is not continuous, classify the type of discontinuity (removable or non-removable) and its location.

(a)

(b)

(c)

(d)

Increasing and Decreasing
A function / If for any
on (a, b) with / In less formal terms:
Increasing / / GOING UP:
a positive change in x results in a positive change in f(x)
Constant / / ALWAYS THE SAME:
a positive change in x results in zero change in f(x)
Decreasing / / GOING DOWN: a positive change in x results in a negative change in f(x)

Example 3: Analyzing a Function for Increasing-Decreasing Behavior

For each function, tell the intervals on which it is increasing and the intervals on which it is decreasing.

(a)

(b)

The next term that can be used to describe function behavior is boundedness.

Boundedness
A function f is…
…bounded below / If there is some number b, such that b is ______. Any such number b is called a lower bound of f.
…bounded above / If there is some number B, such thatB is ______.
Any such number B is called an upper bound of f.
…bounded / If it is bounded both above and below.
…not bounded / If it is not bounded above or below.

Example 4: Determining Boundedness

Identify each of these functions as bounded below, bounded above, bounded or not bounded.

(a) (b) (c) (d)

Many graphs are characterized by peaks and valleys where they change from increasing to decreasing and vice versa. These points are called extrema.

Extrema
Local Maximum / A value f(c) that is ______all range values of f on some open interval containing c.
Local extrema are also called relative extrema.
Absolute Maximum / A value f(c) that is ______all range values of f.
Local Minimum / A value f(c) that is ______all range values of f on some open interval containing c.
Local extrema are also called relative extrema.
Absolute Minimum / A value f(c) that is ______all range values of f.

Example 5: Determining Local Extrema

Determine whether has any local maxima or local minima. If so, find each local maximum or minimum value and the value of x at which each occurs and then determine if any of these values are absolute extrema.

There are two particular types of symmetry that we might look for in a function: symmetry with respect to the y-axis and symmetry with respect to the origin. Generally we are not concerned with symmetry with respect to the x-axis, since graphs with this type of symmetry are not function (with the exception of y = 0).

Symmetry
Graph / Algebra
…with respect to the y-axis /
Functions with this property are called even functions.
…with respect to the origin /
Functions with this property are called odd functions.

Example 6: Determining Symmetry

Tell whether each of the following functions is odd, even, or neither.

(a)

(b)

(c)

Asymptotes

A graph may approach a horizontal or vertical line but never quite reach it. It may also approach a curve (for example, gets awfully close to for large values of x), but the formal definition of an asymptote is again more rigorous than what we require here.

Horizontal asymptotes can be found by simply looking at your graph and seeing if it flattens off towards a certain value as x tends toward positive and negative infinity.

For rational functions,

  • when the degree of the numerator and the denominator are the same, the horizontal asymptote is found by dividing the coefficients of the leading terms.
  • when the degree of the denominator is larger than the degree of the numerator, the horizontal asymptote is the x-axis (y = 0), and
  • when the degree of the denominator is smaller than the degree of the numerator, there is no horizontal asymptote, as there is instead an oblique or slant asymptote.

Vertical asymptotes will most often correspond to the zeroes of the denominator of a rational function.

Example 7: Identifying Asymptotes

Identify any horizontal or vertical asymptotes for the following functions:

(a)

(b)

(c)