In mathematics, a continuous function is a function that does not have any abrupt changes in value, known as discontinuities. More precisely, a function is continuous if arbitrarily small changes in its output can be assured by restricting to sufficiently small changes in its input. If not continuous, a function is said to be discontinuous. Up until the 19th century, mathematicians largely relied on intuitive notions of continuity, during which attempts such as the epsilon–delta definition were made to formalize it.
Continuity of functions is one of the core concepts of topology, which is treated in full generality below. The introductory portion of this article focuses on the special case where the inputs and outputs of functions are real numbers. A stronger form of continuity is uniform continuity. In addition, this article discusses the definition for the more general case of functions between two metric spaces. In order theory, especially in domain theory, one considers a notion of continuity known as Scott continuity. Other forms of continuity do exist but they are not discussed in this article.
As an example, the function H(t) denoting the height of a growing flower at time t would be considered continuous. In contrast, the function M(t) denoting the amount of money in a bank account at time t would be considered discontinuous, since it "jumps" at each point in time when money is deposited or withdrawn.
A form of the epsilon–delta definition of continuity was first given by Bernard Bolzano in 1817. Augustin-Louis Cauchy defined continuity of as follows: an infinitely small increment of the independent variable x always produces an infinitely small change of the dependent variable y (see e.g. Cours d'Analyse, p. 34). Cauchy defined infinitely small quantities in terms of variable quantities, and his definition of continuity closely parallels the infinitesimal definition used today (see microcontinuity). The formal definition and the distinction between pointwise continuity and uniform continuity were first given by Bolzano in the 1830s but the work wasn't published until the 1930s. Like Bolzano, Karl Weierstrass denied continuity of a function at a point c unless it was defined at and on both sides of c, but Édouard Goursat allowed the function to be defined only at and on one side of c, and Camille Jordan allowed it even if the function was defined only at c. All three of those nonequivalent definitions of pointwise continuity are still in use. Eduard Heine provided the first published definition of uniform continuity in 1872, but based these ideas on lectures given by Peter Gustav Lejeune Dirichlet in 1854.
A real function, that is a function from real numbers to real numbers, can be represented by a graph in the Cartesian plane; such a function is continuous if, roughly speaking, the graph is a single unbroken curve whose domain is the entire real line. A more mathematically rigorous definition is given below.
A rigorous definition of continuity of real functions is usually given in a first course in calculus in terms of the idea of a limit. First, a function f with variable x is said to be continuous at the point c on the real line, if the limit of as x approaches that point c, is equal to the value ; and second, the function (as a whole) is said to be continuous, if it is continuous at every point. A function is said to be discontinuous (or to have a discontinuity) at some point when it is not continuous there. These points themselves are also addressed as discontinuities.
There are several different definitions of continuity of a function. Sometimes a function is said to be continuous if it is continuous at every point in its domain. In this case, the function with the domain of all real any integer, is continuous. Sometimes an exception is made for boundaries of the domain. For example, the graph of the function with the domain of all non-negative reals, has a left-hand endpoint. In this case only the limit from the right is required to equal the value of the function. Under this definition f is continuous at the boundary and so for all non-negative arguments. The most common and restrictive definition is that a function is continuous if it is continuous at all real numbers. In this case, the previous two examples are not continuous, but every polynomial function is continuous, as are the sine, cosine, and exponential functions. Care should be exercised in using the word continuous, so that it is clear from the context which meaning of the word is intended.
Using mathematical notation, there are several ways to define continuous functions in each of the three senses mentioned above.
- be a function defined on a subset of the set of real numbers.
This subset is the domain of f. Some possible choices include
- ( is the whole set of real numbers), or, for a and b real numbers,
- ( is a closed interval), or
- ( is an open interval).
In case of the domain being defined as an open interval, and do not belong to , and the values of and do not matter for continuity on .
Definition in terms of limits of functionsEdit
(Here, we have assumed that the domain of f does not have any isolated points.)
Definition in terms of neighborhoodsEdit
A neighborhood of a point c is a set that contains, at least, all points within some fixed distance of c. Intuitively, a function is continuous at a point c if the range of f over the neighborhood of c shrinks to a single point as the width of the neighborhood around c shrinks to zero. More precisely, a function f is continuous at a point c of its domain if, for any neighborhood there is a neighborhood in its domain such that whenever
This definition only requires that the domain and the codomain are topological spaces and is thus the most general definition. It follows from this definition that a function f is automatically continuous at every isolated point of its domain. As a specific example, every real valued function on the set of integers is continuous.
Definition in terms of limits of sequencesEdit
Weierstrass and Jordan definitions (epsilon–delta) of continuous functionsEdit
Explicitly including the definition of the limit of a function, we obtain a self-contained definition: Given a function as above and an element of the domain D, f is said to be continuous at the point when the following holds: For any number however small, there exists some number such that for all x in the domain of f with the value of satisfies
Alternatively written, continuity of at means that for every there exists a such that for all :
More intuitively, we can say that if we want to get all the values to stay in some small neighborhood around we simply need to choose a small enough neighborhood for the x values around If we can do that no matter how small the neighborhood is, then f is continuous at
Weierstrass had required that the interval be entirely within the domain D, but Jordan removed that restriction.
Definition in terms of control of the remainderEdit
In proofs and numerical analysis we often need to know how fast limits are converging, or in other words, control of the remainder. We can formalise this to a definition of continuity. A function is called a control function if
- C is non decreasing
A function is C-continuous at if
A function is continuous in if it is C-continuous for some control function C.
This approach leads naturally to refining the notion of continuity by restricting the set of admissible control functions. For a given set of control functions a function is -continuous if it is -continuous for some For example, the Lipschitz and Hölder continuous functions of exponent α below are defined by the set of control functions
Definition using oscillationEdit
Continuity can also be defined in terms of oscillation: a function f is continuous at a point if and only if its oscillation at that point is zero; in symbols, A benefit of this definition is that it quantifies discontinuity: the oscillation gives how much the function is discontinuous at a point.
This definition is useful in descriptive set theory to study the set of discontinuities and continuous points – the continuous points are the intersection of the sets where the oscillation is less than (hence a set) – and gives a very quick proof of one direction of the Lebesgue integrability condition.
The oscillation is equivalent to the definition by a simple re-arrangement, and by using a limit (lim sup, lim inf) to define oscillation: if (at a given point) for a given there is no that satisfies the definition, then the oscillation is at least and conversely if for every there is a desired the oscillation is 0. The oscillation definition can be naturally generalized to maps from a topological space to a metric space.
Definition using the hyperrealsEdit
Cauchy defined continuity of a function in the following intuitive terms: an infinitesimal change in the independent variable corresponds to an infinitesimal change of the dependent variable (see Cours d'analyse, page 34). Non-standard analysis is a way of making this mathematically rigorous. The real line is augmented by the addition of infinite and infinitesimal numbers to form the hyperreal numbers. In nonstandard analysis, continuity can be defined as follows.
- A real-valued function f is continuous at x if its natural extension to the hyperreals has the property that for all infinitesimal dx, is infinitesimal
(see microcontinuity). In other words, an infinitesimal increment of the independent variable always produces to an infinitesimal change of the dependent variable, giving a modern expression to Augustin-Louis Cauchy's definition of continuity.
Construction of continuous functionsEdit
Checking the continuity of a given function can be simplified by checking one of the above defining properties for the building blocks of the given function. It is straightforward to show that the sum of two functions, continuous on some domain, is also continuous on this domain. Given
The same holds for the product of continuous functions,
In the same way it can be shown that the reciprocal of a continuous function
This implies that, excluding the roots of the quotient of continuous functions
For example, the function (pictured)
Since the function sine is continuous on all reals, the sinc function is defined and continuous for all real However, unlike the previous example, G can be extended to a continuous function on all real numbers, by defining the value to be 1, which is the limit of when x approaches 0, i.e.,
Thus, by setting
the sinc-function becomes a continuous function on all real numbers. The term removable singularity is used in such cases, when (re)defining values of a function to coincide with the appropriate limits make a function continuous at specific points.
A more involved construction of continuous functions is the function composition. Given two continuous functions
This construction allows stating, for example, that
Examples of discontinuous functionsEdit
An example of a discontinuous function is the Heaviside step function , defined by
Pick for instance . Then there is no -neighborhood around , i.e. no open interval with that will force all the values to be within the -neighborhood of , i.e. within . Intuitively we can think of this type of discontinuity as a sudden jump in function values.
Similarly, the signum or sign function
is discontinuous at but continuous everywhere else. Yet another example: the function
is continuous everywhere apart from .
is nowhere continuous.
A useful lemmaEdit
Let be a function that is continuous at a point and be a value such Then throughout some neighbourhood of 
Proof: By the definition of continuity, take , then there exists such that
Intermediate value theoremEdit
- If the real-valued function f is continuous on the closed interval and k is some number between and then there is some number such that
For example, if a child grows from 1 m to 1.5 m between the ages of two and six years, then, at some time between two and six years of age, the child's height must have been 1.25 m.
Extreme value theoremEdit
The extreme value theorem states that if a function f is defined on a closed interval (or any closed and bounded set) and is continuous there, then the function attains its maximum, i.e. there exists with for all The same is true of the minimum of f. These statements are not, in general, true if the function is defined on an open interval (or any set that is not both closed and bounded), as, for example, the continuous function defined on the open interval (0,1), does not attain a maximum, being unbounded above.
Relation to differentiability and integrabilityEdit
Every differentiable function
is everywhere continuous. However, it is not differentiable at (but is so everywhere else). Weierstrass's function is also everywhere continuous but nowhere differentiable.
The derivative of a differentiable function f(x) need not be continuous. If f′(x) is continuous, f(x) is said to be continuously differentiable. The set of such functions is denoted More generally, the set of functions
Every continuous function
Pointwise and uniform limitsEdit
Given a sequence
Directional and semi-continuityEdit
Discontinuous functions may be discontinuous in a restricted way, giving rise to the concept of directional continuity (or right and left continuous functions) and semi-continuity. Roughly speaking, a function is right-continuous if no jump occurs when the limit point is approached from the right. Formally, f is said to be right-continuous at the point c if the following holds: For any number however small, there exists some number such that for all x in the domain with the value of will satisfy
This is the same condition as for continuous functions, except that it is required to hold for x strictly larger than c only. Requiring it instead for all x with yields the notion of left-continuous functions. A function is continuous if and only if it is both right-continuous and left-continuous.
A function f is lower semi-continuous if, roughly, any jumps that might occur only go down, but not up. That is, for any there exists some number such that for all x in the domain with the value of satisfies
Continuous functions between metric spacesEdit
The concept of continuous real-valued functions can be generalized to functions between metric spaces. A metric space is a set X equipped with a function (called metric) that can be thought of as a measurement of the distance of any two elements in X. Formally, the metric is a function
The set of points at which a function between metric spaces is continuous is a set – this follows from the definition of continuity.
Uniform, Hölder and Lipschitz continuityEdit
The concept of continuity for functions between metric spaces can be strengthened in various ways by limiting the way depends on and c in the definition above. Intuitively, a function f as above is uniformly continuous if the does not depend on the point c. More precisely, it is required that for every real number there exists such that for every with we have that Thus, any uniformly continuous function is continuous. The converse does not hold in general, but holds when the domain space X is compact. Uniformly continuous maps can be defined in the more general situation of uniform spaces.
A function is Hölder continuous with exponent α (a real number) if there is a constant K such that for all the inequality
Continuous functions between topological spacesEdit
Another, more abstract, notion of continuity is continuity of functions between topological spaces in which there generally is no formal notion of distance, as there is in the case of metric spaces. A topological space is a set X together with a topology on X, which is a set of subsets of X satisfying a few requirements with respect to their unions and intersections that generalize the properties of the open balls in metric spaces while still allowing to talk about the neighbourhoods of a given point. The elements of a topology are called open subsets of X (with respect to the topology).
An extreme example: if a set X is given the discrete topology (in which every subset is open), all functions
Continuity at a pointEdit
The translation in the language of neighborhoods of the -definition of continuity leads to the following definition of the continuity at a point:
A function is continuous at a point if and only if for any neighborhood V of in Y, there is a neighborhood U of x such that
This definition is equivalent to the same statement with neighborhoods restricted to open neighborhoods and can be restated in several ways by using preimages rather than images.
Also, as every set that contains a neighborhood is also a neighborhood, and is the largest subset U of X such that this definition may be simplified into:
A function is continuous at a point if and only if is a neighborhood of x for every neighborhood V of in Y.
As an open set is a set that is a neighborhood of all its points, a function is continuous at every point of X if and only if it is a continuous function.
If X and Y are metric spaces, it is equivalent to consider the neighborhood system of open balls centered at x and f(x) instead of all neighborhoods. This gives back the above definition of continuity in the context of metric spaces. In general topological spaces, there is no notion of nearness or distance. If however the target space is a Hausdorff space, it is still true that f is continuous at a if and only if the limit of f as x approaches a is f(a). At an isolated point, every function is continuous.
Given a map is continuous at if and only if whenever is a filter on that converges to in which is expressed by writing then necessarily in If denotes the neighborhood filter at then is continuous at if and only if in  Moreover, this happens if and only if the prefilter is a filter base for the neighborhood filter of in 
Several equivalent definitions for a topological structure exist and thus there are several equivalent ways to define a continuous function.
Sequences and nets Edit
In several contexts, the topology of a space is conveniently specified in terms of limit points. In many instances, this is accomplished by specifying when a point is the limit of a sequence, but for some spaces that are too large in some sense, one specifies also when a point is the limit of more general sets of points indexed by a directed set, known as nets. A function is (Heine-)continuous only if it takes limits of sequences to limits of sequences. In the former case, preservation of limits is also sufficient; in the latter, a function may preserve all limits of sequences yet still fail to be continuous, and preservation of nets is a necessary and sufficient condition.
In detail, a function is sequentially continuous if whenever a sequence in X converges to a limit x, the sequence converges to f(x). Thus sequentially continuous functions "preserve sequential limits". Every continuous function is sequentially continuous. If X is a first-countable space and countable choice holds, then the converse also holds: any function preserving sequential limits is continuous. In particular, if X is a metric space, sequential continuity and continuity are equivalent. For non first-countable spaces, sequential continuity might be strictly weaker than continuity. (The spaces for which the two properties are equivalent are called sequential spaces.) This motivates the consideration of nets instead of sequences in general topological spaces. Continuous functions preserve limits of nets, and in fact this property characterizes continuous functions.
For instance, consider the case of real-valued functions of one real variable:
Theorem — A function is continuous at if and only if it is sequentially continuous at that point.
Proof. Assume that is continuous at (in the sense of continuity). Let be a sequence converging at (such a sequence always exists, e.g. ); since is continuous at
Closure operator and interior operator definitionsEdit
In terms of the interior operator, a function between topological spaces is continuous if and only if for every subset
In terms of the closure operator, is continuous if and only if for every subset
Instead of specifying topological spaces by their open subsets, any topology on can alternatively be determined by a closure operator or by an interior operator. Specifically, the map that sends a subset of a topological space to its topological closure satisfies the Kuratowski closure axioms and conversely, for any closure operator there exists a unique topology on (specifically, ) such that for every subset is equal to the topological closure of in If the sets and are each associated with closure operators (both denoted by ) then a map is continuous if and only if for every subset
Similarly, the map that sends a subset of to its topological interior defines an interior operator and conversely, any interior operator induces a unique topology on (specifically, ) such that for every is equal to the topological interior of in If the sets and are each associated with interior operators (both denoted by ) then a map is continuous if and only if for every subset 
Filters and prefiltersEdit
Continuity can also be characterized in terms of filters. A function is continuous if and only if whenever a filter on converges in to a point then the prefilter converges in to This characterization remains true if the word "filter" is replaced by "prefilter."
If and are continuous, then so is the composition If is continuous and
- X is compact, then f(X) is compact.
- X is connected, then f(X) is connected.
- X is path-connected, then f(X) is path-connected.
- X is Lindelöf, then f(X) is Lindelöf.
- X is separable, then f(X) is separable.
The possible topologies on a fixed set X are partially ordered: a topology is said to be coarser than another topology (notation: ) if every open subset with respect to is also open with respect to Then, the identity map
Symmetric to the concept of a continuous map is an open map, for which images of open sets are open. In fact, if an open map f has an inverse function, that inverse is continuous, and if a continuous map g has an inverse, that inverse is open. Given a bijective function f between two topological spaces, the inverse function need not be continuous. A bijective continuous function with continuous inverse function is called a homeomorphism.
Defining topologies via continuous functionsEdit
Given a function
Dually, for a function f from a set S to a topological space X, the initial topology on S is defined by designating as an open set every subset A of S such that for some open subset U of X. If S has an existing topology, f is continuous with respect to this topology if and only if the existing topology is finer than the initial topology on S. Thus the initial topology can be characterized as the coarsest topology on S that makes f continuous. If f is injective, this topology is canonically identified with the subspace topology of S, viewed as a subset of X.
A topology on a set S is uniquely determined by the class of all continuous functions into all topological spaces X. Dually, a similar idea can be applied to maps
If is a continuous function from some subset of a topological space then an continuous extension of to is any continuous function such that for every which is a condition that often written as In words, it is any continuous function that restricts to on This notion is used, for example, in the Tietze extension theorem and the Hahn–Banach theorem. Were not continuous then it could not possibly have a continuous extension. If is a Hausdorff space and is a dense subset of then a continuous extension of to if one exists, will be unique.
Various other mathematical domains use the concept of continuity in different, but related meanings. For example, in order theory, an order-preserving function between particular types of partially ordered sets X and Y is continuous if for each directed subset A of X, we have Here is the supremum with respect to the orderings in X and Y, respectively. This notion of continuity is the same as topological continuity when the partially ordered sets are given the Scott topology.
- Direction-preserving function - an analogue of a continuous function in discrete spaces.
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