discontinuous
Definition
Suppose is an open set in (say an interval , or ), and is a function. Then is discontinuous at , if is not continuous at . One also says that is discontinuous at all boundary points of .
We know that is continuous at if and only if . Thus, from the properties of the one-sided limits, which we denote by and , it follows that is discontinuous at if and only if , or .
If is discontinuous at , the closure of , we can then distinguish four types of different discontinuities as follows [1, 2]:
-
1.
If , but , then is called a removable discontinuity of . If we modify the value of at to , then will become continuous at . This is clear since the modified (call it ) satisfies
-
2.
If , but is not in (so is not defined), then is also called a removable discontinuity. If we assign , then this modification renders continuous at .
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3.
If , then has a jump discontinuity at Then the number is then called the jump, or saltus, of at .
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4.
If either (or both) of or does not exist, then has an essential discontinuity at (or a discontinuity of the second kind).
Note that may be continuous (continuous in all points in ), but still have discontinuities in
Examples
-
1.
Consider the function given by
Since , , and , it follows that has a removable discontinuity at . If we modify so that , then becomes the continuous function .
-
2.
Let us consider the function defined by the formula
where is a nonzero real number. When , the formula is undefined, so is only determined for . Let us show that this point is a removable discontinuity. Indeed, it is easy to see that is continuous for all , and using L’Hôpital’s rule (http://planetmath.org/LHpitalsRule) we have . Thus, if we assign , then becomes a continuous function defined for all real . In fact, can be made into an analytic function on the whole complex plane.
-
3.
The signum function is defined as
Since , , and since , it follows that has a jump discontinuity at with jump .
- 4.
General Definition
Let be topological spaces, and let be a mapping . Then is discontinuous at , if is not continuous at (http://planetmath.org/Continuous) .
In this generality, one generally does not classify discontinuities quite so closely, since they can have quite complicated behaviour.
Notes
A jump discontinuity is also called a simple discontinuity, or a discontinuity of the first kind. An essential discontinuity is also called a discontinuity of the second kind.
References
- 1 R.F. Hoskins, Generalised functions, Ellis Horwood Series: Mathematics and its applications, John Wiley & Sons, 1979.
- 2 P. B. Laval, http://science.kennesaw.edu/ plaval/spring2003/m4400_02/Math4400/contwork.pdfhttp://science.kennesaw.edu/ plaval/spring2003/m4400_02/Math4400/contwork.pdf.
Title | discontinuous |
Canonical name | Discontinuous |
Date of creation | 2013-03-22 13:45:01 |
Last modified on | 2013-03-22 13:45:01 |
Owner | mathwizard (128) |
Last modified by | mathwizard (128) |
Numerical id | 14 |
Author | mathwizard (128) |
Entry type | Definition |
Classification | msc 26A15 |
Classification | msc 54C05 |
Defines | removable discontinuity |
Defines | saltus |
Defines | jump |
Defines | jump discontinuity |
Defines | discontinuity of the second kind |
Defines | discontinuity of the first kind |
Defines | essential discontinuity |