You may want to read this article first: What is a Continuous Function?
A discontinuous function is a function which is not continuous at one or more points.
Being “continuous at every point” means that at every point a:
- The function exists at that point,
- The limit of the function as x goes to the point a exists,
- Both (1) and (2) are equal.
We can write that as:
In plain English, what that means is that the function passes through every point, and each point is close to the next: there are no drastic jumps (see: jump discontinuities). When you’re drawing the graph, you can draw the function without taking your pencil off the paper.
A discontinuous function is one for which you must take the pencil off the paper at least once while drawing.
infinitesimally close. It might also have an asymptote, a line such that, as the function approaches, it goes to infinity. The function never merges with this line, though it may approach infinitely close.
If your function can be written as a fraction, any values of x that make the denominator go to zero will be discontinuities of your function, as at those places your function is not defined.
If you have a piecewise function, the point where one piece ends and another piece ends are also good places to check for discontinuity.
Otherwise, the easiest way to find discontinuities in your function is to graph it. Take note of any holes, any asymptotes, or any jumps. These all represent discontinuities, and just one discontinuity is enough to make your function a discontinuous function.
The four principal types of discontinuity are:
- Jump (Step)
Removable discontinuities are where the limits are equal (it’s a hole, not a jump). It’s called removable because the hole can be filled in. See: Removable Discontinuities.
Infinite discontinuities are when the limit at the gap tends towards infinity. This can be as the function approaches the gap from either the left or the right.
Discontinuous Function: A Note on Classifying Types of Discontinuity
Classifying types of discontinuity is more difficult than it appears, due to the fact that different authors classify them in different ways. For example:
- Some authors simplify the types into two umbrella terms: removable (holes) and non-removable (jumps, infinite and essential discontinuities cannot be removed as they are too far apart or wild in their behavior).
- Essential discontinuities (that jump about wildly as the function approaches the limit) are sometimes referred to as the “non-removable discontinuity”, excluding jumps and infinite from the definition of non-removable.
- Some authors also include “mixed” discontinuities as a type of discontinuity, where the discontinuity is a combination of more than one type.
The takeaway: There isn’t “one” classification system for types of discontinuity that everyone agrees upon. Which system you use will depend upon the text you are using and the preferences of your instructor.
Stephanie Glen. "Discontinuous Function: Types of Discontinuity" From CalculusHowTo.com: Calculus for the rest of us! https://www.calculushowto.com/discontinuous-function/
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