In
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 generalizati ...
, absolute continuity is a
smoothness
In mathematical analysis, the smoothness of a function is a property measured by the number of continuous derivatives it has over some domain, called ''differentiability class''. At the very minimum, a function could be considered smooth if ...
property of
functions that is stronger than
continuity and
uniform continuity
In mathematics, a real function f of real numbers is said to be uniformly continuous if there is a positive real number \delta such that function values over any function domain interval of the size \delta are as close to each other as we want. In ...
. The notion of absolute continuity allows one to obtain generalizations of the relationship between the two central operations 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 generalizati ...
—
differentiation and
integration. This relationship is commonly characterized (by the
fundamental theorem of calculus
The fundamental theorem of calculus is a theorem that links the concept of differentiating a function (calculating its slopes, or rate of change at each time) with the concept of integrating a function (calculating the area under its graph, ...
) in the framework of
Riemann integration, but with absolute continuity it may be formulated in terms of
Lebesgue integration
In mathematics, the integral of a non-negative function of a single variable can be regarded, in the simplest case, as the area between the graph of that function and the -axis. The Lebesgue integral, named after French mathematician Henri Le ...
. For real-valued functions on the
real line
In elementary mathematics, a number line is a picture of a graduated straight line that serves as visual representation of the real numbers. Every point of a number line is assumed to correspond to a real number, and every real number to a po ...
, two interrelated notions appear: absolute continuity of functions and absolute continuity of measures. These two notions are generalized in different directions. The usual derivative of a function is related to the ''
Radon–Nikodym derivative'', or ''density'', of a measure.
We have the following chains of inclusions for functions over a
compact
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in Britis ...
subset of the real line:
: ''
absolutely continuous
In calculus, absolute continuity is a smoothness property of functions that is stronger than continuity and uniform continuity. The notion of absolute continuity allows one to obtain generalizations of the relationship between the two central ope ...
'' ⊆ ''
uniformly continuous
In mathematics, a real function f of real numbers is said to be uniformly continuous if there is a positive real number \delta such that function values over any function domain interval of the size \delta are as close to each other as we want. In ...
''
''
continuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous g ...
''
and, for a compact interval,
: ''
continuously differentiable
In mathematics, a differentiable function of one real variable is a function whose derivative exists at each point in its domain. In other words, the graph of a differentiable function has a non- vertical tangent line at each interior point in ...
'' ⊆ ''
Lipschitz continuous
In mathematical analysis, Lipschitz continuity, named after German mathematician Rudolf Lipschitz, is a strong form of uniform continuity for functions. Intuitively, a Lipschitz continuous function is limited in how fast it can change: there e ...
'' ⊆ ''
absolutely continuous
In calculus, absolute continuity is a smoothness property of functions that is stronger than continuity and uniform continuity. The notion of absolute continuity allows one to obtain generalizations of the relationship between the two central ope ...
'' ⊆ ''
bounded variation'' ⊆ ''
differentiable almost everywhere
In measure theory (a branch of mathematical analysis), a property holds almost everywhere if, in a technical sense, the set for which the property holds takes up nearly all possibilities. The notion of "almost everywhere" is a companion notion to ...
''.
Absolute continuity of functions
A continuous function fails to be absolutely continuous if it fails to be
uniformly continuous
In mathematics, a real function f of real numbers is said to be uniformly continuous if there is a positive real number \delta such that function values over any function domain interval of the size \delta are as close to each other as we want. In ...
, which can happen if the domain of the function is not compact – examples are tan(''x'') over , ''x''
2 over the entire real line, and sin(1/''x'') over (0, 1]. But a continuous function ''f'' can fail to be absolutely continuous even on a compact interval. It may not be "differentiable almost everywhere" (like the
Weierstrass function
In mathematics, the Weierstrass function is an example of a real-valued function that is continuous everywhere but differentiable nowhere. It is an example of a fractal curve. It is named after its discoverer Karl Weierstrass.
The Weierstr ...
, which is not differentiable anywhere). Or it may be
differentiable almost everywhere and its derivative ''f'' ′ may be
Lebesgue integrable
In mathematics, the integral of a non-negative function of a single variable can be regarded, in the simplest case, as the area between the graph of that function and the -axis. The Lebesgue integral, named after French mathematician Henri Leb ...
, but the integral of ''f'' ′ differs from the increment of ''f'' (how much ''f'' changes over an interval). This happens for example with the
Cantor function
In mathematics, the Cantor function is an example of a function that is continuous, but not absolutely continuous. It is a notorious counterexample in analysis, because it challenges naive intuitions about continuity, derivative, and measure. ...
.
Definition
Let
be an
interval in the
real line
In elementary mathematics, a number line is a picture of a graduated straight line that serves as visual representation of the real numbers. Every point of a number line is assumed to correspond to a real number, and every real number to a po ...
. A function
is absolutely continuous on
if for every positive number
, there is a positive number
such that whenever a finite sequence of
pairwise disjoint sub-intervals
of
with
satisfies
:
then
:
The collection of all absolutely continuous functions on
is denoted
.
Equivalent definitions
The following conditions on a real-valued function ''f'' on a compact interval
'a'',''b''are equivalent:
# ''f'' is absolutely continuous;
# ''f'' has a derivative ''f'' ′
almost everywhere
In measure theory (a branch of mathematical analysis), a property holds almost everywhere if, in a technical sense, the set for which the property holds takes up nearly all possibilities. The notion of "almost everywhere" is a companion notion to ...
, the derivative is Lebesgue integrable, and
for all ''x'' on
'a'',''b''
# there exists a Lebesgue integrable function ''g'' on
'a'',''b''such that
for all ''x'' in
'a'',''b''
If these equivalent conditions are satisfied then necessarily ''g'' = ''f'' ′ almost everywhere.
Equivalence between (1) and (3) is known as the fundamental theorem of Lebesgue integral calculus, due to
Lebesgue.
For an equivalent definition in terms of measures see the section
Relation between the two notions of absolute continuity.
Properties
* The sum and difference of two absolutely continuous functions are also absolutely continuous. If the two functions are defined on a bounded closed interval, then their product is also absolutely continuous.
* If an absolutely continuous function is defined on a bounded closed interval and is nowhere zero then its reciprocal is absolutely continuous.
* Every absolutely continuous function (over a compact interval) is
uniformly continuous
In mathematics, a real function f of real numbers is said to be uniformly continuous if there is a positive real number \delta such that function values over any function domain interval of the size \delta are as close to each other as we want. In ...
and, therefore,
continuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous g ...
. Every (globally)
Lipschitz-continuous
In mathematical analysis, Lipschitz continuity, named after German mathematician Rudolf Lipschitz, is a strong form of uniform continuity for functions. Intuitively, a Lipschitz continuous function is limited in how fast it can change: there ...
function is absolutely continuous.
* If ''f'':
'a'',''b''→ R is absolutely continuous, then it is of
bounded variation on
'a'',''b''
* If ''f'':
'a'',''b''→ R is absolutely continuous, then it can be written as the difference of two monotonic nondecreasing absolutely continuous functions on
'a'',''b''
* If ''f'':
'a'',''b''→ R is absolutely continuous, then it has the
Luzin ''N'' property (that is, for any