Continuous functions are of utmost importance in mathematics, functions and applications. However, not all functions are continuous. If a function is not continuous at a limit point (also called "accumulation point" or "cluster point") of its domain, one says that it has a discontinuity there. The set of all points of discontinuity of a function may be a discrete set, a dense set, or even the entire domain of the function. The oscillation of a function at a point quantifies these discontinuities as follows: A special case is if the function diverges to infinity or minus infinity, in which case the oscillation is not defined (in the extended real numbers, this is a removable discontinuity).

Classification

For each of the following, consider a real valued function f of a real variable x, defined in a neighborhood of the point x_0 at which f is discontinuous.

Removable discontinuity

Consider the piecewise function The point x_0 = 1 is a removable discontinuity. For this kind of discontinuity: The one-sided limit from the negative direction: and the one-sided limit from the positive direction: at x_0 both exist, are finite, and are equal to In other words, since the two one-sided limits exist and are equal, the limit L of f(x) as x approaches x_0 exists and is equal to this same value. If**** the actual**** value of**** is**** not equal to**** L,**** then**** x_0 is**** called**** a . This discontinuity can be removed to make f continuous at x_0, or more precisely, the function is continuous at x = x_0. The term removable discontinuity is sometimes broadened to include a removable singularity, in which the limits in both directions exist and are equal, while the function is undefined at the point x_0. This use is an abuse of terminology because continuity and discontinuity of a function are concepts defined only for points in the function's domain.

Jump discontinuity

Consider the function Then, the point x_0 = 1 is a . In this case, a single limit does not exist because the one-sided limits, L^- and L^+ exist and are finite, but are not equal: since, the limit L does not exist. Then, x_0 is called a jump discontinuity, step discontinuity, or discontinuity of the first kind. For this type of discontinuity, the function f may have any value at x_0.

Essential discontinuity

For an essential discontinuity, at least one of the two one-sided limits does not exist in \mathbb{R}. (Notice that one or both one-sided limits can be \pm\infty). Consider the function Then, the point x_0 = 1 is an . In this example, both L^- and L^+ do not exist in \mathbb{R}, thus satisfying the condition of essential discontinuity. So x_0 is an essential discontinuity, infinite discontinuity, or discontinuity of the second kind. (This is distinct from an essential singularity, which is often used when studying functions of complex variables). Supposing that f is a function defined on an interval we will denote by D the set of all discontinuities of f on I. By R we will mean the set of all x_0\in I such that f has a removable discontinuity at x_0. Analogously by J we denote the set constituted by all x_0\in I such that f has a jump discontinuity at x_0. The set of all x_0\in I such that f has an essential discontinuity at x_0 will be denoted by E. Of course then

Counting discontinuities of a function

The two following properties of the set D are relevant in the literature. Tom Apostol follows partially the classification above by considering only removable and jump discontinuities. His objective is to study the discontinuities of monotone functions, mainly to prove Froda’s theorem. With the same purpose, Walter Rudin and Karl R. Stromberg study also removable and jump discontinuities by using different terminologies. However, furtherly, both authors state that R \cup J is always a countable set (see ). The term essential discontinuity has evidence of use in mathematical context as early as 1889. However, the earliest use of the term alongside a mathematical definition seems to have been given in the work by John Klippert. Therein, Klippert also classified essential discontinuities themselves by subdividing the set E into the three following sets: Of course Whenever x_0\in E_1, x_0 is called an essential discontinuity of first kind. Any is said an essential discontinuity of second kind. Hence he enlarges the set R \cup J without losing its characteristic of being countable, by stating the following:

Rewriting Lebesgue's Theorem

When I=[a,b] and f is a bounded function, it is well-known of the importance of the set D in the regard of the Riemann integrability of f. In fact, Lebesgue's Theorem (also named Lebesgue-Vitali) theorem) states that f is Riemann integrable on I = [a,b] if and only if D is a set with Lebesgue's measure zero. In this theorem seems that all type of discontinuities have the same weight on the obstruction that a bounded function f be Riemann integrable on [a,b]. Since countable sets are sets of Lebesgue's measure zero and a countable union of sets with Lebesgue's measure zero is still a set of Lebesgue's mesure zero, we are seeing now that this is not the case. In fact, the discontinuities in the set are absolutely neutral in the regard of the Riemann integrability of f. The main discontinuities for that purpose are the essential discontinuities of first kind and consequently the Lebesgue-Vitali theorem can be rewritten as follows: The case where correspond to the following well-known classical complementary situations of Riemann integrability of a bounded function :

Examples

Thomae's function is discontinuous at every non-zero rational point, but continuous at every irrational point. One easily sees that those discontinuities are all removable. By the first paragraph, there does not exist a function that is continuous at every rational point, but discontinuous at every irrational point. The indicator function of the rationals, also known as the Dirichlet function, is discontinuous everywhere. These discontinuities are all essential of the first kind too. Consider now the ternary Cantor set and its indicator (or characteristic) function One way to construct the Cantor set \mathcal{C} is given by where the sets C_n are obtained by recurrence according to In view of the discontinuities of the function let's assume a point Therefore there exists a set C_n, used in the formulation of \mathcal{C}, which does not contain x_0. That is, x_0 belongs to one of the open intervals which were removed in the construction of C_n. This way, x_0 has a neighbourhood with no points of (In another way, the same conclusion follows taking into account that \mathcal{C} is a closed set and so its complementary with respect to [0, 1] is open). Therefore only assumes the value zero in some neighbourhood of x_0. Hence is continuous at x_0. This means that the set D of all discontinuities of on the interval [0, 1] is a subset of Since \mathcal{C} is an uncountable set with null Lebesgue measure, also D is a null Lebesgue measure set and so in the regard of Lebesgue-Vitali theorem is a Riemann integrable function. More precisely one has In fact, since \mathcal{C} is a nonwhere dense set, if then no neighbourhood of x_0, can be contained in This way, any neighbourhood of contains points of \mathcal{C} and points which are not of In terms of the function this means that both and do not exist. That is, D = E_1, where by E_1, as before, we denote the set of all essential discontinuities of first kind of the function Clearly

Discontinuities of derivatives

Let an open interval, let be differentiable on I, and let be the derivative of F. That is, F'(x)=f(x) for every x\in I. According to Darboux's theorem, the derivative function satisfies the intermediate value property. The function f can, of course, be continuous on the interval I, in which case Bolzano's Theorem also applies. Recall that Bolzano's Theorem asserts that every continuous function satisfies the intermediate value property. On the other hand, the converse is false: Darboux's Theorem does not assume f to be continuous and the intermediate value property does not imply f is continuous on I. Darboux's Theorem does, however, have an immediate consequence on the type of discontinuities that f can have. In fact, if x_0\in I is a point of discontinuity of f, then necessarily x_0 is an essential discontinuity of f. This means in particular that the following two situations cannot occur: Furthermore, two other situations have to be excluded (see John Klippert ): Observe that whenever one of the conditions (i), (ii), (iii), or (iv) is fulfilled for some x_0\in I one can conclude that f fails to possess an antiderivative, F, on the interval I. On the other hand, a new type of discontinuity with respect to any function can be introduced: an essential discontinuity, x_0 \in I, of the function f, is said to be a fundamental essential discontinuity of f if and Therefore if x_0\in I is a discontinuity of a derivative function, then necessarily x_0 is a fundamental essential discontinuity of f. Notice also that when I=[a,b] and is a bounded function, as in the assumptions of Lebesgue's Theorem, we have for all : and Therefore any essential discontinuity of f is a fundamental one.

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