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(fixed spelling of "Nikodym")
 
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{{MSC|28A33}}
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{{MSC|28A15}}
  
 
[[Category:Classical measure theory]]
 
[[Category:Classical measure theory]]
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is denoted by $\nu\ll\mu$. If the measure $\nu$ is finite, i.e. $\nu (X) <\infty$, the property $\nu\ll\mu$ is equivalent
 
is denoted by $\nu\ll\mu$. If the measure $\nu$ is finite, i.e. $\nu (X) <\infty$, the property $\nu\ll\mu$ is equivalent
 
to the following stronger statement: for any $\varepsilon>0$ there is a $\delta>0$ such that $\nu (A)<\varepsilon$ for every
 
to the following stronger statement: for any $\varepsilon>0$ there is a $\delta>0$ such that $\nu (A)<\varepsilon$ for every
$A$ with $\mu (A)<\delta$ (this follows from the Radon-Nykodin theorem, see below, and the absolute continuity of the integral, see for instance
+
$A$ with $\mu (A)<\delta$ (this follows from the Radon-Nikodym theorem, see below, and the absolute continuity of the integral, see for instance
 
Theorem 12.34 of {{Cite|HS}}).
 
Theorem 12.34 of {{Cite|HS}}).
  
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the existence of a function $f\in L^1 (\mu)$ such that $\nu = f \mu$, i.e. such that  
 
the existence of a function $f\in L^1 (\mu)$ such that $\nu = f \mu$, i.e. such that  
 
\[
 
\[
\nu (A) = \int_A f\, \rd\mu \qquad \text{for every $A\in\mathcal{B}$.}
+
\nu (A) = \int_A f\, \rd\mu \qquad \text{for every } A\in\mathcal{B}.
 
\]
 
\]
A corollary of the Radon-Nikodym, the Hahn decomposition theorem (also called Jordan decomposition theorem), characterizes signed measures
+
A corollary of the Radon-Nikodym theorem, the [[Jordan decomposition (of a signed measure)|Jordan decomposition theorem]], characterizes signed measures
as differences of nonnegative measures (see Theorems A and B of Section 29 in {{Cite|Ha}}). We refer to [[Signed measure]] for more on this topic.
+
as differences of nonnegative measures (see Theorems A and B of Section 29 in {{Cite|Ha}}). We refer to [[Signed measure]] for more on this topic. See also [[Hahn decomposition]].
  
 
Two measures which are mutually absolutely continuous are sometimes called equivalent.
 
Two measures which are mutually absolutely continuous are sometimes called equivalent.
  
====Radon-Nikdoym decomposition====
+
====Radon-Nikodym decomposition====
 
If $\mu$ is a $\sigma$-finite nonnegative measure on a $\sigma$-algebra $\mathcal{B}$ and $\nu$ another $\sigma$-finite nonnegative measure on the same $\sigma$-algebra (which might be a signed measure, or even taking values in a finite-dimensional
 
If $\mu$ is a $\sigma$-finite nonnegative measure on a $\sigma$-algebra $\mathcal{B}$ and $\nu$ another $\sigma$-finite nonnegative measure on the same $\sigma$-algebra (which might be a signed measure, or even taking values in a finite-dimensional
 
vector space), then $\nu$ can be decomposed in a unique way as $\nu=\nu_a+\nu_s$ where
 
vector space), then $\nu$ can be decomposed in a unique way as $\nu=\nu_a+\nu_s$ where
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The same decomposition holds even if $\nu$ is a [[Signed measure|signed measure]] or, more generally, a vector-valued
 
The same decomposition holds even if $\nu$ is a [[Signed measure|signed measure]] or, more generally, a vector-valued
 
measure. In these cases the property $\nu_s (X\setminus A)=0$ is substituted by $\left|\nu_s\right| (X\setminus A)=0$, where $\left|\nu_s\right|$ denotes the total variation measure of $\nu_s$ (we refer to [[Signed measure]] for the relevant definition).
 
measure. In these cases the property $\nu_s (X\setminus A)=0$ is substituted by $\left|\nu_s\right| (X\setminus A)=0$, where $\left|\nu_s\right|$ denotes the total variation measure of $\nu_s$ (we refer to [[Signed measure]] for the relevant definition).
 +
 +
Some authors use the name "Differentiation of measures" for the decomposition above and the density $f$ is sometimes denoted by $\frac{d\nu}{d\mu}$ or $D_\mu \nu$ (see for instance Section 32 of {{Cite|Ha}}). Other authors use the term "Differentiation of measures" for a theorem, due to Besicovitch, which, for [[Radon measure|Radon measures]] in the Euclidean space, characterizes $f(x)$ as the limit of a suitable quantity, see [[Differentiation of measures]] for the precise statement.
  
 
====Comments====
 
====Comments====
A set of non-zero measure that has no subsets of smaller, but still positive, measure is called an atom of the measure. When considering
+
A set of non-zero measure that has no subsets of smaller, but still positive, measure is called an [[Atom|atom]] of the measure. When considering
 
the $\sigma$-algebra $\mathcal{B}$ of [[Borel set|Borel sets]] in the euclidean space and the Lebesgue measure $\lambda$ as reference measure, it is a common mistake to claim that the singular part of a second measure $\nu$ must be concentrated on points which are atoms. A singular measure may be atomless, as is shown by the measure concentrated on the standard [[Cantor set|Cantor set]] which puts zero on each gap of the set and $2^{-n}$ on the intersection of the set with the interval of generation $n$ (such measure is also the [[Generalized derivative|distributional derivative]]
 
the $\sigma$-algebra $\mathcal{B}$ of [[Borel set|Borel sets]] in the euclidean space and the Lebesgue measure $\lambda$ as reference measure, it is a common mistake to claim that the singular part of a second measure $\nu$ must be concentrated on points which are atoms. A singular measure may be atomless, as is shown by the measure concentrated on the standard [[Cantor set|Cantor set]] which puts zero on each gap of the set and $2^{-n}$ on the intersection of the set with the interval of generation $n$ (such measure is also the [[Generalized derivative|distributional derivative]]
of the Cantor ternary function or devil staircase, (see Problem 46 in Chapter 2 of {{Cite|Ro}}).  
+
of the [[Cantor ternary function]] or devil staircase, (see Problem 46 in Chapter 2 of {{Cite|Ro}}).  
  
 
When some canonical measure $\mu$ is fixed, (as the [[Lebesgue measure|Lebesgue measure]] on $\mathbb R^n$ or its subsets or, more generally, the [[Haar measure|Haar measure]] on a [[Topological group|topological group]]), one says that $\nu$ is absolutely continuous meaning that $\nu\ll\mu$.
 
When some canonical measure $\mu$ is fixed, (as the [[Lebesgue measure|Lebesgue measure]] on $\mathbb R^n$ or its subsets or, more generally, the [[Haar measure|Haar measure]] on a [[Topological group|topological group]]), one says that $\nu$ is absolutely continuous meaning that $\nu\ll\mu$.
 
For Radon measures in the Euclidean space, both the Radon-Nikodym theorem and the Radon-Nikodym decomposition can be proved using the Besicovitch [[Differentiation of measures|differentiation of measures]].
 
 
  
 
====References====
 
====References====
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|-
 
|-
 
|valign="top"|{{Ref|AFP}}||    L. Ambrosio, N. Fusco, D. Pallara, "Functions of bounded variations    and  free discontinuity problems". Oxford Mathematical Monographs. The    Clarendon Press, Oxford University Press, New York, 2000.    {{MR|1857292}}{{ZBL|0957.49001}}  
 
|valign="top"|{{Ref|AFP}}||    L. Ambrosio, N. Fusco, D. Pallara, "Functions of bounded variations    and  free discontinuity problems". Oxford Mathematical Monographs. The    Clarendon Press, Oxford University Press, New York, 2000.    {{MR|1857292}}{{ZBL|0957.49001}}  
 +
|-
 +
|valign="top"|{{Ref|Bi}}||    P. Billingsley, "Convergence  of probability measures", Wiley (1968)    {{MR|0233396}}  {{ZBL|0172.21201}}
 
|-
 
|-
 
|valign="top"|{{Ref|Bo}}||    N. Bourbaki, "Elements of mathematics. Integration", Addison-Wesley    (1975) pp. Chapt.6;7;8 (Translated from French) {{MR|0583191}}    {{ZBL|1116.28002}} {{ZBL|1106.46005}} {{ZBL|1106.46006}}    {{ZBL|1182.28002}} {{ZBL|1182.28001}} {{ZBL|1095.28002}}    {{ZBL|1095.28001}} {{ZBL|0156.06001}}
 
|valign="top"|{{Ref|Bo}}||    N. Bourbaki, "Elements of mathematics. Integration", Addison-Wesley    (1975) pp. Chapt.6;7;8 (Translated from French) {{MR|0583191}}    {{ZBL|1116.28002}} {{ZBL|1106.46005}} {{ZBL|1106.46006}}    {{ZBL|1182.28002}} {{ZBL|1182.28001}} {{ZBL|1095.28002}}    {{ZBL|1095.28001}} {{ZBL|0156.06001}}
 
|-
 
|-
 
|valign="top"|{{Ref|DS}}||    N. Dunford, J.T. Schwartz, "Linear operators. General theory",    '''1''', Interscience (1958) {{MR|0117523}} {{ZBL|0635.47001}}
 
|valign="top"|{{Ref|DS}}||    N. Dunford, J.T. Schwartz, "Linear operators. General theory",    '''1''', Interscience (1958) {{MR|0117523}} {{ZBL|0635.47001}}
|-
 
|valign="top"|{{Ref|Bi}}||    P. Billingsley, "Convergence of probability measures", Wiley (1968)    {{MR|0233396}} {{ZBL|0172.21201}}
 
 
|-
 
|-
 
|valign="top"|{{Ref|Ha}}|| P.R. Halmos,  "Measure theory", v. Nostrand  (1950) {{MR|0033869}} {{ZBL|0040.16802}}
 
|valign="top"|{{Ref|Ha}}|| P.R. Halmos,  "Measure theory", v. Nostrand  (1950) {{MR|0033869}} {{ZBL|0040.16802}}

Latest revision as of 07:56, 15 December 2016

2020 Mathematics Subject Classification: Primary: 28A15 [MSN][ZBL]

A concept in measure theory (see also Absolute continuity). If $\mu$ and $\nu$ are two measures on a σ-algebra $\mathcal{B}$ of subsets of $X$, we say that $\nu$ is absolutely continuous with respect to $\mu$ if $\nu (A) =0$ for any $A\in\mathcal{B}$ such that $\mu (A) =0$ (cp. with Defininition 2.11 of [Ma]). The absolute continuity of $\nu$ with respect to $\mu$ is denoted by $\nu\ll\mu$. If the measure $\nu$ is finite, i.e. $\nu (X) <\infty$, the property $\nu\ll\mu$ is equivalent to the following stronger statement: for any $\varepsilon>0$ there is a $\delta>0$ such that $\nu (A)<\varepsilon$ for every $A$ with $\mu (A)<\delta$ (this follows from the Radon-Nikodym theorem, see below, and the absolute continuity of the integral, see for instance Theorem 12.34 of [HS]).

This definition can be generalized to signed measures $\nu$ and even to vector-valued measures $\nu$. Some authors generalize it further to vector-valued $\mu$'s: in that case the absolute continuity of $\nu$ with respect to $\mu$ amounts to the requirement that $\nu (A) = 0$ for any $A\in\mathcal{B}$ such that $|\mu| (A)=0$, where $|\mu|$ is the total variation of $\mu$ (see for instance Theorem B, Section 31 of [Ha]).

Under the assumption that $\mu$ is $\sigma$-finite, the Radon-Nikodym theorem (see Theorem B of Section 31 in [Ha]) characterizes the absolute continuity of $\nu$ with respect to $\mu$ with the existence of a function $f\in L^1 (\mu)$ such that $\nu = f \mu$, i.e. such that \[ \nu (A) = \int_A f\, \rd\mu \qquad \text{for every } A\in\mathcal{B}. \] A corollary of the Radon-Nikodym theorem, the Jordan decomposition theorem, characterizes signed measures as differences of nonnegative measures (see Theorems A and B of Section 29 in [Ha]). We refer to Signed measure for more on this topic. See also Hahn decomposition.

Two measures which are mutually absolutely continuous are sometimes called equivalent.

Radon-Nikodym decomposition

If $\mu$ is a $\sigma$-finite nonnegative measure on a $\sigma$-algebra $\mathcal{B}$ and $\nu$ another $\sigma$-finite nonnegative measure on the same $\sigma$-algebra (which might be a signed measure, or even taking values in a finite-dimensional vector space), then $\nu$ can be decomposed in a unique way as $\nu=\nu_a+\nu_s$ where

  • $\nu_a$ is absolutely continuous with respect to $\mu$;
  • $\nu_s$ is singular with respect to $\mu$, i.e. there is a set $A$ of $\mu$-measure zero such that $\nu_s (X\setminus A)=0$ (this property is often denoted by $\nu_s\perp \mu$).

This decomposition is called Radon-Nikodym decomposition by some authors and Lebesgue decomposition by some other (see Theorem C of Section 32 in [Ha]). The same decomposition holds even if $\nu$ is a signed measure or, more generally, a vector-valued measure. In these cases the property $\nu_s (X\setminus A)=0$ is substituted by $\left|\nu_s\right| (X\setminus A)=0$, where $\left|\nu_s\right|$ denotes the total variation measure of $\nu_s$ (we refer to Signed measure for the relevant definition).

Some authors use the name "Differentiation of measures" for the decomposition above and the density $f$ is sometimes denoted by $\frac{d\nu}{d\mu}$ or $D_\mu \nu$ (see for instance Section 32 of [Ha]). Other authors use the term "Differentiation of measures" for a theorem, due to Besicovitch, which, for Radon measures in the Euclidean space, characterizes $f(x)$ as the limit of a suitable quantity, see Differentiation of measures for the precise statement.

Comments

A set of non-zero measure that has no subsets of smaller, but still positive, measure is called an atom of the measure. When considering the $\sigma$-algebra $\mathcal{B}$ of Borel sets in the euclidean space and the Lebesgue measure $\lambda$ as reference measure, it is a common mistake to claim that the singular part of a second measure $\nu$ must be concentrated on points which are atoms. A singular measure may be atomless, as is shown by the measure concentrated on the standard Cantor set which puts zero on each gap of the set and $2^{-n}$ on the intersection of the set with the interval of generation $n$ (such measure is also the distributional derivative of the Cantor ternary function or devil staircase, (see Problem 46 in Chapter 2 of [Ro]).

When some canonical measure $\mu$ is fixed, (as the Lebesgue measure on $\mathbb R^n$ or its subsets or, more generally, the Haar measure on a topological group), one says that $\nu$ is absolutely continuous meaning that $\nu\ll\mu$.

References

[AFP] L. Ambrosio, N. Fusco, D. Pallara, "Functions of bounded variations and free discontinuity problems". Oxford Mathematical Monographs. The Clarendon Press, Oxford University Press, New York, 2000. MR1857292Zbl 0957.49001
[Bi] P. Billingsley, "Convergence of probability measures", Wiley (1968) MR0233396 Zbl 0172.21201
[Bo] N. Bourbaki, "Elements of mathematics. Integration", Addison-Wesley (1975) pp. Chapt.6;7;8 (Translated from French) MR0583191 Zbl 1116.28002 Zbl 1106.46005 Zbl 1106.46006 Zbl 1182.28002 Zbl 1182.28001 Zbl 1095.28002 Zbl 1095.28001 Zbl 0156.06001
[DS] N. Dunford, J.T. Schwartz, "Linear operators. General theory", 1, Interscience (1958) MR0117523 Zbl 0635.47001
[Ha] P.R. Halmos, "Measure theory", v. Nostrand (1950) MR0033869 Zbl 0040.16802
[HS] E. Hewitt, K.R. Stromberg, "Real and abstract analysis", Springer (1965) MR0188387 Zbl 0137.03202
[Ro] H.L. Royden, "Real analysis", Macmillan (1969) MR0151555 Zbl 0197.03501
How to Cite This Entry:
Absolutely continuous measures. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Absolutely_continuous_measures&oldid=27497
This article was adapted from an original article by T. Nowicki (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article