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### Determining Whether a Function is Continuous

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Date: 06/10/2000 at 22:05:02
From: John
Subject: Continuous functions

How can you tell whether or not a function is continuous?
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Date: 06/13/2000 at 18:11:10
From: Doctor Maureen
Subject: Re: Continuous functions

Dear John,

Welcome to the Math Forum and thanks for your interesting question.
You can approach this question graphically or algebraically (that is,
by analyzing the function without looking at its graph).

I'll address continuity from a graphical standpoint first. A function
is continuous if you can sketch the entire graph without lifting your
pencil from the paper. In other words, the graph has no breaks in it.
One example is the graph of a parabola, f(x) = x^2 + 1. The graph for
this function is continuous because you can plot a y-value for every
possible x-value, and the y-values have no "sudden jumps," so the
graph is a smooth continuous graph. An example of a discontinuous
graph is g(x) = 1/x. Since the variable is in the denominator, g(x) is
not defined for x = 0. When you look at the graph for this function,
you will see an asymptote at x = 0, which is the y-axis. When you try
to sketch this graph, you may plot a few points and then attempt to
connect them into one curve. However, as you approach zero from the
negative direction, you will have to stop at x = 0, lift your pencil
and start again at the lowest positive number.

You might be wondering, what if I can't see the graph of the function
- how will I know if it is continuous? Well there are really only two
kinds of functions that you will have to analyze for continuity,
rational functions in which there is a fraction and the variable is in
the denominator, and piecewise functions. Here are some examples of
rational functions:

f(x) = 2/(x-1)
g(x) = (x+3)/(x^2-4)
h(x) = 5/(x^3+1)

In each of these functions, x is in the denominator and there is a
value or values for x that will make the function undefined. Do you
see that for h(x)? If x = -1, the denominator will be 0. Therefore,
h is discontinuous at -1. For what values of x are f(x) and g(x)
discontinuous? In these cases the points of discontinuity are directly
related to the domain. For h(x), the domain is all real numbers except
-1. For f(x), the domain is all real numbers except 1. So the
functions are discontinuous at the points that are not defined in the
domain.

This is not always the case. Consider the function

f(x) = sqrt(x-5)

(Sqrt stands for square root - I can't type the radical symbol into
this e-mail.) In this case, f is defined for all real numbers greater
than or equal to 5 since you can't take the square root of a negative
number. Even though the domain of f is limited, f is considered
continuous along its defined values. Its curve starts at (5,0) and
goes on to infinity without a break in the graph. It is only rational
functions that have a direct relationship between the points of
discontinuity and their domain.

The other type of function is the piecewise function, in which a
function has two or more definitions depending of the value of x. For
example, consider g(x) = x+1 if x < 0, and g(x) = x^2 if x >= 0. Can
you picture that g(x) is a straight line from negative infinity until
x = 0? Then at 0, g turns into a parabola for the positive x values.
At x = 0, the straight-line portion of the graph will end at the point
(0,1), and at (0,0) the parabola will start. So there will be a break in
the curve; your pencil will not be able to continue the curve without
lifting at (0,1) and then starting the parabola at (0,0). When
analyzing a piecewise function, you must determine what happens at the
point(s) where the definition changes. Sometimes the two definitions
overlap and there is a continuous graph, other times there is a gap
between the two curves. In the example I just described, do you see
that if I changed the second definition for g to g(x) = x^2 + 1 then
the two graphs would overlap at (0,1)? Thus, g would be continuous.

My explanation got a bit lengthy. To summarize, continuous functions
are functions in which the graphs are smooth curves or lines without
breaks in them. The two types of functions that may have points of
discontinuity are rational and piecewise functions. Rational functions
will be discontinuous at the points where the function is undefined.
Piecewise functions may be discontinuous at the points where the
function definition changes. In the latter, you will see a gap in the
graph.

I hope this explanation helps. Please write back if I used language
that is unfamiliar to you.

- Doctor Maureen, The Math Forum
http://mathforum.org/dr.math/
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Associated Topics:
High School Calculus
High School Equations, Graphs, Translations

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