TheInfoList

In
mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and their changes (cal ...
, a continuous function is a
function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
such that a continuous variation (that is a change without jump) of the
argument In logic Logic is an interdisciplinary field which studies truth and reasoning Reason is the capacity of consciously making sense of things, applying logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, lab ...
induces a continuous variation of the
value Value or values may refer to: * Value (ethics) In ethics Ethics or moral philosophy is a branch of philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about Metaphysics, existence, reason, E ...
of the function. This means that there is no abrupt changes in value, known as discontinuities. More precisely, a function is continuous if arbitrarily small changes in its value can be assured by restricting to sufficiently small changes of its argument. A discontinuous function is a function that is . Up until the 19th century, mathematicians largely relied on
intuitive Intuition is the ability to acquire knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge), skills (procedural knowledge), or objects (Knowledge by acquaintance, acqu ...

notions of continuity, and considered only continuous functions. The epsilon–delta definition of a limit was introduced to formalize the definition of continuity. Continuity is one of the core concepts of
calculus Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematics, mathematical study of continuous change, in the same way that geometry is the study of shape and algebra is the study of generalizations ...

and
mathematical analysis Analysis is the branch of mathematics dealing with Limit (mathematics), limits and related theories, such as Derivative, differentiation, Integral, integration, Measure (mathematics), measure, sequences, Series (mathematics), series, and analytic ...
, where arguments and values of functions are
real Real may refer to: * Reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only Object of the mind, imaginary. The term is also used to refer to the ontological status of things, ind ...
and
complex The UCL Faculty of Mathematical and Physical Sciences is one of the 11 constituent faculties of University College London , mottoeng = Let all come who by merit deserve the most reward , established = , type = Public university, Public rese ...

numbers. The concept has been generalized to functions between
metric space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
s and between
topological space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gener ...
s. The latter are the most general continuous functions, and their definition is the basis of
topology In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities ...

. A stronger form of continuity is
uniform continuity In mathematics, a function (mathematics), function ''f'' is uniformly continuous if, roughly speaking, it is possible to guarantee that ''f''(''x'') and ''f''(''y'') be as close to each other as we please by requiring only that ''x'' and ''y'' be s ...
. In
order theory Order theory is a branch of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geom ...
, especially in
domain theory Domain theory is a branch of mathematics that studies special kinds of partially ordered sets (posets) commonly called domains. Consequently, domain theory can be considered as a branch of order theory. The field has major applications in computer s ...
, a related concept of continuity is
Scott continuityIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
. As an example, the function denoting the height of a growing flower at time would be considered continuous. In contrast, the function denoting the amount of money in a bank account at time would be considered discontinuous, since it "jumps" at each point in time when money is deposited or withdrawn.

# History

A form of the was first given by
Bernard Bolzano Bernard Bolzano (, ; ; ; born Bernardus Placidus Johann Nepomuk Bolzano; 5 October 1781 – 18 December 1848) was a Bohemian A Bohemian () is a resident of Bohemia Bohemia ( ; cs, Čechy ; ; hsb, Čěska; szl, Czechy) is the westernmost a ...

in 1817.
Augustin-Louis Cauchy Baron Baron is a rank of nobility or title of honour, often hereditary, in various European countries, either current or historical. The female equivalent is baroness. Typically, the title denotes an aristocrat who ranks higher than a lord ...

defined continuity of $y = f\left(x\right)$ as follows: an infinitely small increment $\alpha$ of the independent variable ''x'' always produces an infinitely small change $f\left(x+\alpha\right)-f\left(x\right)$ of the dependent variable ''y'' (see e.g. ''
Cours d'Analyse ''Cours d'Analyse de l’École Royale Polytechnique; I.re Partie. Analyse algébrique'' is a seminal textbook in infinitesimal calculus published by Augustin-Louis Cauchy in 1821. The article follows the translation by Bradley and Sandifer in de ...
'', 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
microcontinuityIn nonstandard analysis The history of calculus is fraught with philosophical debates about the meaning and logical validity of fluxions or infinitesimal numbers. The standard way to resolve these debates is to define the operations of calculus ...
). The formal definition and the distinction between pointwise continuity and
uniform continuity In mathematics, a function (mathematics), function ''f'' is uniformly continuous if, roughly speaking, it is possible to guarantee that ''f''(''x'') and ''f''(''y'') be as close to each other as we please by requiring only that ''x'' and ''y'' be s ...
were first given by Bolzano in the 1830s but the work wasn't published until the 1930s. Like Bolzano,
Karl Weierstrass Karl Theodor Wilhelm Weierstrass (german: link=no, Weierstraß ; 31 October 1815 – 19 February 1897) was a German mathematics, mathematician often cited as the "father of modern mathematical analysis, analysis". Despite leaving university withou ...

denied continuity of a function at a point ''c'' unless it was defined at and on both sides of ''c'', but
Édouard Goursat Édouard Jean-Baptiste Goursat (21 May 1858 – 25 November 1936) was a French French (french: français(e), link=no) may refer to: * Something of, from, or related to France France (), officially the French Republic (french: link=no, Ré ...
allowed the function to be defined only at and on one side of ''c'', and
Camille Jordan Marie Ennemond Camille Jordan (; 5 January 1838 – 22 January 1922) was a French mathematician, known both for his foundational work in group theory and for his influential ''Cours d'analyse''. Biography Jordan was born in Lyon and educated at ...
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 Heinrich Eduard Heine (16 March 1821 – October 1881) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of German ...
provided the first published definition of uniform continuity in 1872, but based these ideas on lectures given by
Peter Gustav Lejeune Dirichlet Johann Peter Gustav Lejeune Dirichlet (; 13 February 1805 – 5 May 1859) was a German mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study ...

in 1854.

# Real functions

## Definition

A
real function In mathematical analysis, and applications in geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. ...
, that is a
function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
from
real number In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
s to real numbers, can be represented by a
graph Graph may refer to: Mathematics *Graph (discrete mathematics), a structure made of vertices and edges **Graph theory, the study of such graphs and their properties *Graph (topology), a topological space resembling a graph in the sense of discret ...

in the
Cartesian plane A Cartesian coordinate system (, ) in a plane Plane or planes may refer to: * Airplane or aeroplane or informally plane, a powered, fixed-wing aircraft Arts, entertainment and media *Plane (Dungeons & Dragons), Plane (''Dungeons & Dragons'') ...
; such a function is continuous if, roughly speaking, the graph is a single unbroken
curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line (geometry), line, but that does not have to be Linearity, straight. Intuitively, a curve may be thought of as the trace left by a moving point (geo ...

whose
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
is the entire real line. A more mathematically rigorous definition is given below. Continuity of real functions is usually defined in terms of
limits Limit or Limits may refer to: Arts and media * Limit (music) In music theory, limit or harmonic limit is a way of characterizing the harmony found in a piece or genre (music), genre of music, or the harmonies that can be made using a particular ...
. A function with variable is ''continuous at'' the
real number In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
, if the limit of $f\left(x\right),$ as tends to , is equal to $f\left(c\right).$ There are several different definitions of (global) continuity of a function, which depend on the nature of its
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
. A function is continuous on an
open interval In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ge ...
if the interval is contained in the domain of the function, and the function is continuous at every point of the interval. A function that is continuous on the interval $\left(-\infty, +\infty\right)$ (the whole
real line In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
) is often called simply a continuous function; one says also that such a function is ''continuous everywhere''. For example, all
polynomial function In mathematics, a polynomial is an expression (mathematics), expression consisting of variable (mathematics), variables (also called indeterminate (variable), indeterminates) and coefficients, that involves only the operations of addition, subtra ...
s are continuous everywhere. A function is continuous on a semi-open or a closed interval, if the interval is contained in the domain of the function, the function is continuous at every interior point of the interval, and the value of the function at each endpoint that belongs to the interval is the limit of the values of the function when the variable tends to the endpoint from the interior of the interval. For example, the function $f\left(x\right) = \sqrt$ is continuous on its whole domain, which is the closed interval $\left[0,+\infty\right).$ Many commonly encountered functions have a domain formed by all
real number In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
s, except some
isolated point 400px, "0" is an isolated point of A = ∪ , 2In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus ...
s. Examples are the functions $x\mapsto \frac 1x$ and $x\mapsto \tan x.$ When they are continuous on their domain, one says, in some contexts, that they are continuous, although they are not continuous everywhere. In other contexts, mainly when one is interested with their behavior near the exceptional points, one says that they are discontinuous. A function is ''discontinuous'' at a point, if the point belongs to the
topological closure In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
of its domain, and either the point does not belong to the domain of the function, or the function is not continuous at the point. For example, the functions $x\mapsto \frac 1x$ and $x\mapsto \sin(\frac 1x)$ are discontinuous at , and remain discontinuous whichever value is chosen for defining them at . A point where a function is discontinuous is called a ''discontinuity''. Using mathematical notation, there are several ways to define continuous functions in each of the three senses mentioned above. Let :$f : D \to \R \quad$ be a function defined on a
subset In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

$D$ of the set $\R$ of real numbers. This subset $D$ is the
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
of ''f''. Some possible choices include :$D = \R \quad$ ($D$ is the whole set of real numbers), or, for ''a'' and ''b'' real numbers, :$D =$
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
= \ \quad ($D$ is a
closed interval In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
), or :$D = \left(a, b\right) = \ \quad$ ($D$ is an
open interval In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ge ...
). In case of the domain $D$ being defined as an open interval, $a$ and $b$ do not belong to $D$, and the values of $f\left(a\right)$ and $f\left(b\right)$ do not matter for continuity on $D$.

### Definition in terms of limits of functions

The function ''f'' is ''continuous at some point'' ''c'' of its domain if the
limit Limit or Limits may refer to: Arts and media * Limit (music), a way to characterize harmony * Limit (song), "Limit" (song), a 2016 single by Luna Sea * Limits (Paenda song), "Limits" (Paenda song), 2019 song that represented Austria in the Eurov ...

of $f\left(x\right),$ as ''x'' approaches ''c'' through the domain of ''f'', exists and is equal to $f\left(c\right).$ In mathematical notation, this is written as $\lim_ = f(c).$ In detail this means three conditions: first, ''f'' has to be defined at ''c'' (guaranteed by the requirement that ''c'' is in the domain of ''f''). Second, the limit on the left hand side of that equation has to exist. Third, the value of this limit must equal $f\left(c\right).$ (Here, we have assumed that the domain of ''f'' does not have any
isolated point 400px, "0" is an isolated point of A = ∪ , 2In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus ...
s.)

### Definition in terms of neighborhoods

A
neighborhood A neighbourhood (British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar ...
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 $f\left(c\right)$ 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 $N_1\left(f\left(c\right)\right)$ there is a neighborhood $N_2\left(c\right)$ in its domain such that $f\left(x\right) \in N_1\left(f\left(c\right)\right)$ whenever $x\in N_2\left(c\right).$ 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 400px, "0" is an isolated point of A = ∪ , 2In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus ...
of its domain. As a specific example, every real valued function on the set of integers is continuous.

### Definition in terms of limits of sequences

One can instead require that for any
sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
$\left(x_n\right)_$ of points in the domain which converges to ''c'', the corresponding sequence $\left\left(f\left(x_n\right)\right\right)_$ converges to $f\left(c\right).$ In mathematical notation, $\forall \left(x_n\right)_ \subset D:\lim_ x_n = c \Rightarrow \lim_ f\left(x_n\right) = f\left(c\right)\,.$

### Weierstrass and Jordan definitions (epsilon–delta) of continuous functions

Explicitly including the definition of the limit of a function, we obtain a self-contained definition: Given a function $f : D \to R$ as above and an element $x_0$ of the domain ''D'', ''f'' is said to be continuous at the point $x_0$ when the following holds: For any number $\varepsilon > 0,$ however small, there exists some number $\delta > 0$ such that for all ''x'' in the domain of ''f'' with $x_0 - \delta < x < x_0 + \delta,$ the value of $f\left(x\right)$ satisfies $f\left(x_0\right) - \varepsilon < f(x) < f(x_0) + \varepsilon.$ Alternatively written, continuity of $f : D \to R$ at $x_0 \in D$ means that for every $\varepsilon > 0,$ there exists a $\delta > 0$ such that for all $x \in D$: $\left, x - x_0\ < \delta \text , f(x) - f(x_0), < \varepsilon.$ More intuitively, we can say that if we want to get all the $f\left(x\right)$ values to stay in some small
neighborhood A neighbourhood (British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar ...
around $f\left\left(x_0\right\right),$ we simply need to choose a small enough neighborhood for the ''x'' values around $x_0.$ If we can do that no matter how small the $f\left(x\right)$ neighborhood is, then ''f'' is continuous at $x_0.$ In modern terms, this is generalized by the definition of continuity of a function with respect to a basis for the topology, here the
metric topology In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
. Weierstrass had required that the interval $x_0 - \delta < x < x_0 + \delta$ be entirely within the domain ''D'', but Jordan removed that restriction.

### Definition in terms of control of the remainder

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

### Definition using oscillation

Continuity can also be defined in terms of
oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of Mechanical equilibrium, equilibrium) or between two or more different states. The term ''vibration'' is precisely used to describ ...
: a function ''f'' is continuous at a point $x_0$ if and only if its oscillation at that point is zero; in symbols, $\omega_f\left(x_0\right) = 0.$ A benefit of this definition is that it discontinuity: the oscillation gives how the function is discontinuous at a point. This definition is useful in
descriptive set theory In mathematical logic Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical pr ...
to study the set of discontinuities and continuous points – the continuous points are the intersection of the sets where the oscillation is less than $\varepsilon$ (hence a $G_$ set) – and gives a very quick proof of one direction of the Lebesgue integrability condition. The oscillation is equivalent to the $\varepsilon-\delta$ 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 $\varepsilon_0$ there is no $\delta$ that satisfies the $\varepsilon-\delta$ definition, then the oscillation is at least $\varepsilon_0,$ and conversely if for every $\varepsilon$ there is a desired $\delta,$ the oscillation is 0. The oscillation definition can be naturally generalized to maps from a topological space to a metric space.

### Definition using the hyperreals

Cauchy Baron Augustin-Louis Cauchy (; ; 21 August 178923 May 1857) was a French mathematician, engineer, and physicist who made pioneering contributions to several branches of mathematics, including mathematical analysis and continuum mechanics. He was ...

defined continuity of a function in the following intuitive terms: an
infinitesimal In mathematics, infinitesimals or infinitesimal numbers are quantities that are closer to zero than any standard real number, but are not zero. They do not exist in the standard real number system, but do exist in many other number systems, such a ...
change in the independent variable corresponds to an infinitesimal change of the dependent variable (see ''Cours d'analyse'', page 34).
Non-standard analysis The history of calculus is fraught with philosophical debates about the meaning and logical validity of fluxions or infinitesimal In mathematics, infinitesimals or infinitesimal numbers are quantities that are closer to zero than any standard ...
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 mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
. 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'', $f\left(x + dx\right) - f\left(x\right)$ is infinitesimal (see
microcontinuityIn nonstandard analysis The history of calculus is fraught with philosophical debates about the meaning and logical validity of fluxions or infinitesimal numbers. The standard way to resolve these debates is to define the operations of calculus ...
). 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 Baron Baron is a rank of nobility or title of honour, often hereditary, in various European countries, either current or historical. The female equivalent is baroness. Typically, the title denotes an aristocrat who ranks higher than a lord ...

's definition of continuity.

## Construction of continuous functions

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 $f, g \colon D \to \R,$ then the $s = f + g$ (defined by $s\left(x\right) = f\left(x\right) + g\left(x\right)$ for all $x\in D$) is continuous in $D.$ The same holds for the , $p = f \cdot g$ (defined by $p\left(x\right) = f\left(x\right) \cdot g\left(x\right)$ for all $x \in D$) is continuous in $D.$ Combining the above preservations of continuity and the continuity of
constant function 270px, Constant function ''y''=4 In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (m ...

s and of the
identity function Graph of the identity function on the real numbers In mathematics, an identity function, also called an identity relation, identity map or identity transformation, is a function (mathematics), function that always returns the same value that was ...

$I\left(x\right) = x$ one arrives at the continuity of all
polynomial function In mathematics, a polynomial is an expression (mathematics), expression consisting of variable (mathematics), variables (also called indeterminate (variable), indeterminates) and coefficients, that involves only the operations of addition, subtra ...

s such as $f(x) = x^3 + x^2 - 5 x + 3$ (pictured on the right). In the same way it can be shown that the $r = 1/f$ (defined by $r\left(x\right) = 1/f\left(x\right)$ for all $x \in D$ such that $f\left(x\right) \neq 0$) is continuous in $D\setminus \.$ This implies that, excluding the roots of $g,$ the $q = f / g$ (defined by $q\left(x\right) = f\left(x\right)/g\left(x\right)$ for all $x \in D$, such that $g\left(x\right) \neq 0$) is also continuous on $D\setminus \$. For example, the function (pictured) $y(x) = \frac$ is defined for all real numbers $x \neq -2$ and is continuous at every such point. Thus it is a continuous function. The question of continuity at $x = -2$ does not arise, since $x = -2$ is not in the domain of $y.$ There is no continuous function $F : \R \to \R$ that agrees with $y\left(x\right)$ for all $x \neq -2.$ Since the function
sine In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

is continuous on all reals, the
sinc function In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ge ...

$G\left(x\right) = \sin\left(x\right)/x,$ is defined and continuous for all real $x \neq 0.$ However, unlike the previous example, ''G'' be extended to a continuous function on real numbers, by the value $G\left(0\right)$ to be 1, which is the limit of $G\left(x\right),$ when ''x'' approaches 0, i.e., $G(0) = \lim_ \frac = 1.$ Thus, by setting :$G\left(x\right) = \begin \frac x & \textx \ne 0\\ 1 & \textx = 0, \end$ the sinc-function becomes a continuous function on all real numbers. The term 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 In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
. Given two continuous functions $g : D_g \subseteq \R \to R_g \subseteq \R \quad \text \quad f : D_f \subseteq \R \to R_f \subseteq D_g,$ their composition, denoted as $c = g \circ f : D_f \to \R,$ and defined by $c\left(x\right) = g\left(f\left(x\right)\right),$ is continuous. This construction allows stating, for example, that $e^$ is continuous for all $x > 0.$

## Examples of discontinuous functions

An example of a discontinuous function is the
Heaviside step function 325px, The Heaviside step function, using the half-maximum convention The Heaviside step function, or the unit step function, usually denoted by or (but sometimes , or ), is a step function, named after Oliver Heaviside (1850–1925), the valu ...
$H$, defined by :$H\left(x\right) = \begin 1 & \text x \ge 0\\ 0 & \text x < 0 \end$ Pick for instance $\varepsilon = 1/2$. Then there is no around $x = 0$, i.e. no open interval $\left(-\delta,\;\delta\right)$ with $\delta > 0,$ that will force all the $H\left(x\right)$ values to be within the of $H\left(0\right)$, i.e. within $\left(1/2,\;3/2\right)$. Intuitively we can think of this type of discontinuity as a sudden jump in function values. Similarly, the signum or sign function :$\sgn\left(x\right) = \begin \;\;\ 1 & \textx > 0\\ \;\;\ 0 & \textx = 0\\ -1 & \textx < 0 \end$ is discontinuous at $x = 0$ but continuous everywhere else. Yet another example: the function :$f\left(x\right) = \begin \sin\left\left(x^\right\right)&\textx \neq 0\\ 0&\textx = 0 \end$ is continuous everywhere apart from $x = 0$. Besides plausible continuities and discontinuities like above, there are also functions with a behavior, often coined
pathological Pathology is the study of the causesCauses, or causality, is the relationship between one event and another. It may also refer to: * Causes (band), an indie band based in the Netherlands * Causes (company), an online company See also * Cau ...
, for example,
Thomae's function Thomae's function, named after Carl Johannes Thomae, has many names: the popcorn function, the raindrop function, the countable cloud function, the modified Dirichlet function, the ruler function, the Riemann function, or the Stars over Babylon (J ...
, :$f\left(x\right)=\begin 1 &\text x=0\\ \frac&\text x = \frac \text\\ 0&\textx\text. \end$ is continuous at all irrational numbers and discontinuous at all rational numbers. In a similar vein, Dirichlet's function, the
indicator function Image:Indicator function illustration.png, A three-dimensional plot of an indicator function, shown over a square two-dimensional domain (set ): the 'raised' portion overlays those two-dimensional points which are members of the 'indicated' subs ...
for the set of rational numbers, :$D\left(x\right)=\begin 0&\textx\text \left(\in \R \setminus \Q\right)\\ 1&\textx\text \left(\in \Q\right) \end$ is nowhere continuous.

## Properties

### A useful lemma

Let $f\left(x\right)$ be a function that is continuous at a point $x_0,$ and $y_0$ be a value such $f\left\left(x_0\right\right)\neq y_0.$ Then $f\left(x\right)\neq y_0$ throughout some neighbourhood of $x_0.$ ''Proof:'' By the definition of continuity, take $\varepsilon =\frac>0$ , then there exists $\delta>0$ such that $\left, f(x)-f(x_0)\ < \frac \quad \text \quad , x-x_0, < \delta$ Suppose there is a point in the neighbourhood $, x-x_0, <\delta$ for which $f\left(x\right)=y_0;$ then we have the contradiction $\left, f(x_0)-y_0\ < \frac.$

### Intermediate value theorem

The
intermediate value theorem In mathematical analysis, the intermediate value theorem states that if ''f'' is a continuous function whose domain contains the interval 'a'', ''b'' then it takes on any given value between ''f''(''a'') and ''f''(''b'') at some point ...

is an
existence theorem In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
, based on the real number property of completeness, and states: :If the real-valued function ''f'' is continuous on the
closed interval In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...

, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
and ''k'' is some number between $f\left(a\right)$ and $f\left(b\right),$ then there is some number $c \in$
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
such that $f\left(c\right) = k.$ 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. As a consequence, if ''f'' is continuous on 
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
/math> and $f\left(a\right)$ and $f\left(b\right)$ differ in
sign A sign is an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the grasp of the senses ** Object (abstract), an object which does not exist at ...
, then, at some point $c \in$
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
$f\left(c\right)$ must equal
zero 0 (zero) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) formally defined, and ...

.

### Extreme value theorem

The
extreme value theorem 300px, A continuous function f(x) on the closed interval _showing_the_absolute_max_(red)_and_the_absolute_min_(blue)..html" ;"title=",b/math> showing the absolute max (red) and the absolute min (blue).">,b/math> showing the absolute max (red) an ...
states that if a function ''f'' is defined on a closed interval 
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
/math> (or any closed and bounded set) and is continuous there, then the function attains its maximum, i.e. there exists $c \in$
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
/math> with $f\left(c\right) \geq f\left(x\right)$ for all $x \in$
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
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 $\left(a, b\right)$ (or any set that is not both closed and bounded), as, for example, the continuous function $f\left(x\right) = \frac,$ defined on the open interval (0,1), does not attain a maximum, being unbounded above.

### Relation to differentiability and integrability

Every
differentiable function In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...

$f : (a, b) \to \R$ is continuous, as can be shown. The
converse Converse may refer to: Mathematics and logic * Converse (logic), the result of reversing the two parts of a categorical or implicational statement ** Converse implication, the converse of a material implication ** Converse nonimplication, a logical ...

does not hold: for example, the
absolute value In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities an ...

function :$f\left(x\right)=, x, = \begin \;\;\ x & \textx \geq 0\\ -x & \textx < 0 \end$ is everywhere continuous. However, it is not differentiable at $x = 0$ (but is so everywhere else). is also everywhere continuous but nowhere differentiable. The
derivative In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities ...

$f\left(x\right)$ 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 $C^1\left(\left(1, b\right)\right).$ More generally, the set of functions $f : \Omega \to \R$ (from an open interval (or
open subset Open or OPEN may refer to: Music * Open (band) Open is a band. Background Drummer Pete Neville has been involved in the Sydney/Australian music scene for a number of years. He has recently completed a Masters in screen music at the Australian ...
of $\R$) $\Omega$ to the reals) such that ''f'' is $n$ times differentiable and such that the $n$-th derivative of ''f'' is continuous is denoted $C^n\left(\Omega\right).$ See
differentiability class In calculus (a branch of mathematics), a differentiable function of one Real number, real variable is a function whose derivative exists at each point in its Domain of a function, domain. In other words, the Graph of a function, graph of a differen ...
. In the field of computer graphics, properties related (but not identical) to $C^0, C^1, C^2$ are sometimes called $G^0$ (continuity of position), $G^1$ (continuity of tangency), and $G^2$ (continuity of curvature); see Smoothness of curves and surfaces. Every continuous function $f :$
, b The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
\to \R is
integrableIn mathematics, integrability is a property of certain dynamical system In mathematics, a dynamical system is a system in which a Function (mathematics), function describes the time dependence of a Point (geometry), point in a Manifold, geometric ...
(for example in the sense of the
Riemann integral The partition does not need to be regular, as shown here. The approximation works as long as the width of each subdivision tends to zero. In the branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such to ...
). The converse does not hold, as the (integrable, but discontinuous)
sign function In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...
shows.

### Pointwise and uniform limits

Given a
sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
$f_1, f_2, \dotsc : I \to \R$ of functions such that the limit $f(x) := \lim_ f_n(x)$ exists for all $x \in D,$, the resulting function $f\left(x\right)$ is referred to as the pointwise limit of the sequence of functions $\left\left(f_n\right\right)_.$ The pointwise limit function need not be continuous, even if all functions $f_n$ are continuous, as the animation at the right shows. However, ''f'' is continuous if all functions $f_n$ are continuous and the sequence converges uniformly, by the uniform convergence theorem. This theorem can be used to show that the
exponential function The exponential function is a mathematical function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of ...

s,
logarithm In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ...

s,
square root In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities ...

function, and
trigonometric function In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...

s are continuous.

## Directional and semi-continuity

Image:Right-continuous.svg, A right-continuous function Image:Left-continuous.svg, A left-continuous function
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 In mathematical analysis, semi-continuity (or semicontinuity) is a property of extended real-valued functions that is weaker than continuity. An extended real-valued function f is upper (respectively, lower) semi-continuous at a point x_0 if, ...
. Roughly speaking, a function is 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 $\varepsilon > 0$ however small, there exists some number $\delta > 0$ such that for all ''x'' in the domain with $c < x < c + \delta,$ the value of $f\left(x\right)$ will satisfy $, f(x) - f(c), < \varepsilon.$ 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 $c - \delta < x < c$ yields the notion of functions. A function is continuous if and only if it is both right-continuous and left-continuous. A function ''f'' is if, roughly, any jumps that might occur only go down, but not up. That is, for any $\varepsilon > 0,$ there exists some number $\delta > 0$ such that for all ''x'' in the domain with $, x - c, < \delta,$ the value of $f\left(x\right)$ satisfies $f(x) \geq f(c) - \epsilon.$ The reverse condition is .

# Continuous functions between metric spaces

The concept of continuous real-valued functions can be generalized to functions between
metric space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
s. A metric space is a set ''X'' equipped with a function (called
metric METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) is a computer model Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of or th ...
) $d_X,$ that can be thought of as a measurement of the distance of any two elements in ''X''. Formally, the metric is a function $d_X : X \times X \to \R$ that satisfies a number of requirements, notably the
triangle inequality In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities an ...

. Given two metric spaces $\left\left(X, d_X\right\right)$ and $\left\left(Y, d_Y\right\right)$ and a function $f : X \to Y$ then ''f'' is continuous at the point $c \in X$ (with respect to the given metrics) if for any positive real number $\varepsilon > 0,$ there exists a positive real number $\delta > 0$ such that all $x \in X$ satisfying $d_X\left(x, c\right) < \delta$ will also satisfy $d_Y\left(f\left(x\right), f\left(c\right)\right) < \varepsilon.$ As in the case of real functions above, this is equivalent to the condition that for every sequence $\left\left(x_n\right\right)$ in ''X'' with limit $\lim x_n = c,$ we have $\lim f\left\left(x_n\right\right) = f\left(c\right).$ The latter condition can be weakened as follows: ''f'' is continuous at the point ''c'' if and only if for every convergent sequence $\left\left(x_n\right\right)$ in ''X'' with limit ''c'', the sequence $\left\left(f\left\left(x_n\right\right)\right\right)$ is a
Cauchy sequence In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
, and ''c'' is in the domain of ''f''. The set of points at which a function between metric spaces is continuous is a $G_$ set – this follows from the $\varepsilon-\delta$ definition of continuity. This notion of continuity is applied, for example, in
functional analysis 200px, One of the possible modes of vibration of an idealized circular drum head. These modes are eigenfunctions of a linear operator on a function space, a common construction in functional analysis. Functional analysis is a branch of mathemat ...
. A key statement in this area says that a
linear operator In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
$T : V \to W$ between
normed vector space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
s ''V'' and ''W'' (which are
vector space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...
s equipped with a compatible
norm Norm, the Norm or NORM may refer to: In academic disciplines * Norm (geology), an estimate of the idealised mineral content of a rock * Norm (philosophy) Norms are concepts ( sentences) of practical import, oriented to effecting an action, rat ...
, denoted $\, x\,$) is continuous if and only if it is bounded, that is, there is a constant ''K'' such that $\, T(x)\, \leq K \, x\,$ for all $x \in V.$

## Uniform, Hölder and Lipschitz continuity

The concept of continuity for functions between metric spaces can be strengthened in various ways by limiting the way $\delta$ depends on $\varepsilon$ and ''c'' in the definition above. Intuitively, a function ''f'' as above is
uniformly continuous In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
if the $\delta$ does not depend on the point ''c''. More precisely, it is required that for every
real number In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
$\varepsilon > 0$ there exists $\delta > 0$ such that for every $c, b \in X$ with $d_X\left(b, c\right) < \delta,$ we have that $d_Y\left(f\left(b\right), f\left(c\right)\right) < \varepsilon.$ Thus, any uniformly continuous function is continuous. The converse does not hold in general, but holds when the domain space ''X'' is
compact Compact as used in politics may refer broadly to a pact A pact, from Latin ''pactum'' ("something agreed upon"), is a formal agreement. In international relations International relations (IR), international affairs (IA) or internationa ...
. Uniformly continuous maps can be defined in the more general situation of
uniform space In the mathematical field of topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathemati ...
s. A function is
Hölder continuousHölder: * ''Hölder, Hoelder'' as surname * Hölder condition In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry ...
with exponent α (a real number) if there is a constant ''K'' such that for all $b, c \in X,$ the inequality $d_Y (f(b), f(c)) \leq K \cdot (d_X (b, c))^\alpha$ holds. Any Hölder continuous function is uniformly continuous. The particular case $\alpha = 1$ is referred to as
Lipschitz continuity In mathematical analysis, Lipschitz continuity, named after Rudolf Lipschitz, is a strong form of uniform continuity for function (mathematics), functions. Intuitively, a Lipschitz continuous function is limited in how fast it can change: there ...

. That is, a function is Lipschitz continuous if there is a constant ''K'' such that the inequality $d_Y (f(b), f(c)) \leq K \cdot d_X (b, c)$ holds for any $b, c \in X.$ The Lipschitz condition occurs, for example, in the
Picard–Lindelöf theorem In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
concerning the solutions of
ordinary differential equation In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
s.

# Continuous functions between topological spaces

Another, more abstract, notion of continuity is continuity of functions between
topological space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gener ...
s in which there generally is no formal notion of distance, as there is in the case of
metric space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
s. A topological space is a set ''X'' together with a topology on ''X'', which is a set of
subset In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

s of ''X'' satisfying a few requirements with respect to their unions and intersections that generalize the properties of the
open ball In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s in metric spaces while still allowing to talk about the neighbourhoods of a given point. The elements of a topology are called
open subset Open or OPEN may refer to: Music * Open (band) Open is a band. Background Drummer Pete Neville has been involved in the Sydney/Australian music scene for a number of years. He has recently completed a Masters in screen music at the Australian ...
s of ''X'' (with respect to the topology). A function $f : X \to Y$ between two topological spaces ''X'' and ''Y'' is continuous if for every open set $V \subseteq Y,$ the
inverse image In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities an ...

$f^(V) = \$ is an open subset of ''X''. That is, ''f'' is a function between the sets ''X'' and ''Y'' (not on the elements of the topology $T_X$), but the continuity of ''f'' depends on the topologies used on ''X'' and ''Y''. This is equivalent to the condition that the of the
closed set In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position of ...
s (which are the complements of the open subsets) in ''Y'' are closed in ''X''. An extreme example: if a set ''X'' is given the
discrete topology In topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric objec ...
(in which every subset is open), all functions $f : X \to T$ to any topological space ''T'' are continuous. On the other hand, if ''X'' is equipped with the
indiscrete topologyIn topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric object ...
(in which the only open subsets are the empty set and ''X'') and the space ''T'' set is at least T0, then the only continuous functions are the constant functions. Conversely, any function whose range is indiscrete is continuous.

The translation in the language of neighborhoods of the $\left(\varepsilon, \delta\right)$-definition of continuity leads to the following definition of the continuity at a point: This definition is equivalent to the same statement with neighborhoods restricted to open neighborhoods and can be restated in several ways by using
preimage In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
s rather than images. Also, as every set that contains a neighborhood is also a neighborhood, and $f^\left(V\right)$ is the largest subset of such that $f\left(U\right) \subseteq V,$ this definition may be simplified into: As an open set is a set that is a neighborhood of all its points, a function $f : X \to Y$ is continuous at every point of if and only if it is a continuous function. If ''X'' and ''Y'' are metric spaces, it is equivalent to consider the
neighborhood systemIn topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric object ...
of
open ball In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s centered at ''x'' and ''f''(''x'') instead of all neighborhoods. This gives back the above $\varepsilon-\delta$ 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 In topology In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), ...

, 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 $x \in X,$ a map $f : X \to Y$ is continuous at $x$ if and only if whenever $\mathcal$ is a filter on $X$ that converges to $x$ in $X,$ which is expressed by writing $\mathcal \to x,$ then necessarily $f\left(\mathcal\right) \to f\left(x\right)$ in $Y.$ If $\mathcal\left(x\right)$ denotes the neighborhood filter at $x$ then $f : X \to Y$ is continuous at $x$ if and only if $f\left(\mathcal\left(x\right)\right) \to f\left(x\right)$ in $Y.$ Moreover, this happens if and only if the prefilter $f\left(\mathcal\left(x\right)\right)$ is a filter base for the neighborhood filter of $f\left(x\right)$ in $Y.$

## Alternative definitions

Several Characterizations of the category of topological spaces, equivalent definitions for a topological structure exist and thus there are several equivalent ways to define a continuous function.

### Sequences and nets

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 family, indexed by a directed set, known as Net (mathematics), 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 $f : X \to Y$ is Sequential continuity, sequentially continuous if whenever a sequence $\left\left(x_n\right\right)$ in $X$ converges to a limit $x,$ the sequence $\left\left(f\left\left(x_n\right\right)\right\right)$ converges to $f\left(x\right).$ Thus sequentially continuous functions "preserve sequential limits". Every continuous function is sequentially continuous. If $X$ is a first-countable space and Axiom of countable choice, 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: ''Proof.'' Assume that $f : A \subseteq \R \to \R$ is continuous at $x_0$ (in the sense of (ε, δ)-definition of limit#Continuity, $\epsilon-\delta$ continuity). Let $\left\left(x_n\right\right)_$ be a sequence converging at $x_0$ (such a sequence always exists, for example, $x_n = x, \text n$); since $f$ is continuous at $x_0$ $\forall \epsilon > 0\, \exists \delta_ > 0 : 0 < , x-x_0, < \delta_ \implies , f(x)-f(x_0), < \epsilon.\quad (*)$ For any such $\delta_$ we can find a natural number $\nu_ > 0$ such that for all $n > \nu_,$ $, x_n-x_0, < \delta_,$ since $\left\left(x_n\right\right)$ converges at $x_0$; combining this with $\left(*\right)$ we obtain $\forall \epsilon > 0 \,\exists \nu_ > 0 : \forall n > \nu_ \quad , f(x_n)-f(x_0), < \epsilon.$ Assume on the contrary that $f$ is sequentially continuous and proceed by contradiction: suppose $f$ is not continuous at $x_0$ $\exists \epsilon > 0 : \forall \delta_ > 0,\,\exists x_: 0 < , x_-x_0, < \delta_\epsilon \implies , f(x_)-f(x_0), > \epsilon$ then we can take $\delta_=1/n,\,\forall n > 0$ and call the corresponding point $x_ =: x_n$: in this way we have defined a sequence $\left(x_n\right)_$ such that $\forall n > 0 \quad , x_n-x_0, < \frac,\quad , f(x_n)-f(x_0), > \epsilon$ by construction $x_n \to x_0$ but $f\left(x_n\right) \not\to f\left(x_0\right)$, which contradicts the hypothesis of sequentially continuity. $\blacksquare$

### Closure operator and interior operator definitions

In terms of the Interior (topology), interior operator, a function $f : X \to Y$ between topological spaces is continuous if and only if for every subset $B \subseteq Y,$ $f^\left(\operatorname_Y B\right) ~\subseteq~ \operatorname_X\left(f^(B)\right).$ In terms of the Closure (topology), closure operator, $f : X \to Y$ is continuous if and only if for every subset $A \subseteq X,$ $f\left(\operatorname_X A\right) ~\subseteq~ \operatorname_Y (f(A)).$ That is to say, given any element $x \in X$ that belongs to the closure of a subset $A \subseteq X,$ $f\left(x\right)$ necessarily belongs to the closure of $f\left(A\right)$ in $Y.$ If we declare that a point $x$ is a subset $A \subseteq X$ if $x \in \operatorname_X A,$ then this terminology allows for a plain English description of continuity: $f$ is continuous if and only if for every subset $A \subseteq X,$ $f$ maps points that are close to $A$ to points that are close to $f\left(A\right).$ Similarly, $f$ is continuous at a fixed given point $x \in X$ if and only if whenever $x$ is close to a subset $A \subseteq X,$ then $f\left(x\right)$ is close to $f\left(A\right).$ Instead of specifying topological spaces by their Open set, open subsets, any topology on $X$ can Equivalence of categories, alternatively be determined by a Kuratowski closure operator, closure operator or by an interior operator. Specifically, the map that sends a subset $A$ of a topological space $X$ to its Closure (topology), topological closure $\operatorname_X A$ satisfies the Kuratowski closure axioms. Conversely, for any Kuratowski closure operator, closure operator $A \mapsto \operatorname A$ there exists a unique topology $\tau$ on $X$ (specifically, $\tau := \$) such that for every subset $A \subseteq X,$ $\operatorname A$ is equal to the topological closure $\operatorname_ A$ of $A$ in $\left(X, \tau\right).$ If the sets $X$ and $Y$ are each associated with closure operators (both denoted by $\operatorname$) then a map $f : X \to Y$ is continuous if and only if $f\left(\operatorname A\right) \subseteq \operatorname \left(f\left(A\right)\right)$ for every subset $A \subseteq X.$ Similarly, the map that sends a subset $A$ of $X$ to its Interior (topology), topological interior $\operatorname_X A$ defines an interior operator. Conversely, any interior operator $A \mapsto \operatorname A$ induces a unique topology $\tau$ on $X$ (specifically, $\tau := \$) such that for every $A \subseteq X,$ $\operatorname A$ is equal to the topological interior $\operatorname_ A$ of $A$ in $\left(X, \tau\right).$ If the sets $X$ and $Y$ are each associated with interior operators (both denoted by $\operatorname$) then a map $f : X \to Y$ is continuous if and only if $f^\left(\operatorname B\right) \subseteq \operatorname\left\left(f^\left(B\right)\right\right)$ for every subset $B \subseteq Y.$

### Filters and prefilters

Continuity can also be characterized in terms of Filter (set theory), filters. A function $f : X \to Y$ is continuous if and only if whenever a filter $\mathcal$ on $X$ converges in $X$ to a point $x \in X,$ then the prefilter $f\left(\mathcal\right)$ converges in $Y$ to $f\left(x\right).$ This characterization remains true if the word "filter" is replaced by "prefilter."

## Properties

If $f : X \to Y$ and $g : Y \to Z$ are continuous, then so is the composition $g \circ f : X \to Z.$ If $f : X \to Y$ is continuous and * ''X'' is Compact space, compact, then ''f''(''X'') is compact. * ''X'' is Connected space, connected, then ''f''(''X'') is connected. * ''X'' is path-connected, then ''f''(''X'') is path-connected. * ''X'' is Lindelöf space, Lindelöf, then ''f''(''X'') is Lindelöf. * ''X'' is separable space, separable, then ''f''(''X'') is separable. The possible topologies on a fixed set ''X'' are partial ordering, partially ordered: a topology $\tau_1$ is said to be Comparison of topologies, coarser than another topology $\tau_2$ (notation: $\tau_1 \subseteq \tau_2$) if every open subset with respect to $\tau_1$ is also open with respect to $\tau_2.$ Then, the identity function, identity map $\operatorname_X : \left(X, \tau_2\right) \to \left(X, \tau_1\right)$ is continuous if and only if $\tau_1 \subseteq \tau_2$ (see also comparison of topologies). More generally, a continuous function $\left(X, \tau_X\right) \to \left(Y, \tau_Y\right)$ stays continuous if the topology $\tau_Y$ is replaced by a Comparison of topologies, coarser topology and/or $\tau_X$ is replaced by a Comparison of topologies, finer topology.

## Homeomorphisms

Symmetric to the concept of a continuous map is an open map, for which 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 $f^$ need not be continuous. A bijective continuous function with continuous inverse function is called a . If a continuous bijection has as its
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
a compact space and its codomain is Hausdorff space, Hausdorff, then it is a homeomorphism.

## Defining topologies via continuous functions

Given a function $f : X \to S,$ where ''X'' is a topological space and ''S'' is a set (without a specified topology), the final topology on ''S'' is defined by letting the open sets of ''S'' be those subsets ''A'' of ''S'' for which $f^\left(A\right)$ is open in ''X''. If ''S'' has an existing topology, ''f'' is continuous with respect to this topology if and only if the existing topology is Comparison of topologies, coarser than the final topology on ''S''. Thus the final topology can be characterized as the finest topology on ''S'' that makes ''f'' continuous. If ''f'' is surjective, this topology is canonically identified with the quotient topology under the equivalence relation defined by ''f''. 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 $A = f^\left(U\right)$ 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 $S \to X$ into all topological spaces ''X''. Duality (mathematics), Dually, a similar idea can be applied to maps $X \to S.$

# Related notions

If $f : S \to Y$ is a continuous function from some subset $S$ of a topological space $X$ then an of $f$ to $X$ is any continuous function $F : X \to Y$ such that $F\left(s\right) = f\left(s\right)$ for every $s \in S,$ which is a condition that often written as $f = F\big\vert_S.$ In words, it is any continuous function $F : X \to Y$ that Restriction of a function, restricts to $f$ on $S.$ This notion is used, for example, in the Tietze extension theorem and the Hahn–Banach theorem. Were $f : S \to Y$ not continuous then it could not possibly have a continuous extension. If $Y$ is a
Hausdorff space In topology In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), ...

and $S$ is a Dense set, dense subset of $X$ then a continuous extension of $f : S \to Y$ to $X,$ 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 Order theory is a branch of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geom ...
, an order-preserving function $f : X \to Y$ between particular types of partially ordered sets ''X'' and ''Y'' is Scott continuity, continuous if for each Directed set, directed subset ''A'' of ''X'', we have $\sup f\left(A\right) = f\left(\sup A\right).$ Here $\,\sup\,$ 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. In category theory, a functor $F : \mathcal C \to \mathcal D$ between two Category (mathematics), categories is called , if it commutes with small Limit (category theory), limits. That is to say, $\varprojlim_ F(C_i) \cong F \left(\varprojlim_ C_i \right)$ for any small (i.e., indexed by a set ''I'', as opposed to a class (mathematics), class) Diagram (category theory), diagram of Object (category theory), objects in $\mathcal C$. A is a generalization of metric spaces and posets, which uses the concept of quantales, and that can be used to unify the notions of metric spaces and Domain theory, domains.