Difference between revisions of "Carathéodory measure"
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− | + | {{MSC|28A}} | |
− | + | [[Category:Classical measure theory]] | |
− | + | {{TEX|done}} | |
− | + | The term might refer to different objects in classical measure theory. | |
+ | ====Caratheodory measures and outer measures in metric spaces==== | ||
+ | Consider an [[Outer measure|outer measure]] $\mu$ defined on the class $\mathcal{P} (X)$ of subsets of a metric space $(X,d)$. $\mu$ is a ''Caratheodory outer measure'', more often called ''metric outer measure'' (cp. with Section 11 of {{Cite|Ha}}), if | ||
+ | \begin{equation}\label{e:additive} | ||
+ | \mu (A\cup B) = \mu (A) + \mu (B) | ||
+ | \end{equation} | ||
+ | for every pair of sets $A, B\subset X$ which have positive distance (i.e. such that $\inf \{d(x,y): x\in A, y\in B\} > 0$). A theorem due to Caratheodory shows then that the Borel sets are $\mu$-measurable (see [[Outer measure#Caratheodory criterion]], also for the notion of $\mu$-measurability). The restriction of $\mu$ to the [[Algebra of sets|$\sigma$-algebra]] of $\mu$-measurable sets is called, by some authors, the Caratheodory measure induced by the metric outer measure $\mu$. | ||
− | + | The converse is also true: if $\mu$ is an outer measure on a metric space $(X,d)$ for which the open set are $\mu$-measurable, then $\mu$ is a metric outer measure (see for instance Remark (8c) of Section 11 in {{Cite|Ha}}). | |
− | |||
− | |||
− | |||
+ | ====Caratheodory outer measures with respect to a class of functions==== | ||
+ | More generally, given a set $X$ and a class $\Gamma$ of real functions on $X$, some authors (see for instance Section 7 of Chapter 12 in {{Cite|Ro}}) call ''Caratheodory outer measures with respect to $\Gamma$'' those outer measures $\mu$ on $\mathcal{P} (X)$ with the property that \eqref{e:additive} holds when $A$ and $B$ are ''separated by $\Gamma$'', i.e. when there is a function $\varphi\in \Gamma$ with $\inf_A\; \varphi > \sup_B \varphi$ or $\inf_B\;\varphi > \sup_B\; \varphi$. | ||
+ | If $(X,d)$ is a metric space and we chose as $\Gamma$ the set of functions of type $x\mapsto {\rm dist}\, (x, E)$ with $E\subset X$, then a ''Caratheodory outer measure with respect to $\Gamma$'' corresponds to a Caratheodory outer measure in the sense of the previous section. | ||
− | ==== | + | ====Caratheodory (outer) measures in the Euclidean space==== |
− | + | Some authors use the term ''Caratheodory (outer) measures'' for a special class of outer measures defined on the subsets of the euclidean space $\mathbb R^n$ and constructed in a fashion similar to the usual [[Hausdorff measure|Hausdorff (outer) measures]]. Cp. for instance with Sections 2.1.3-2.1.4-2.1.5 of {{Cite|KP}} and Sections 2.10.2-2.10.3-2.10.4 of {{Cite|Fe}}. | |
====References==== | ====References==== | ||
− | + | {| | |
+ | |- | ||
+ | |valign="top"|{{Ref|Ca}}|| C. Carathéodory, "Über das lineare Mass von Punktmengen, eine Verallgemeinerung des Längenbegriffs" ''Nachr. Gesell. Wiss. Göttingen'' (1914) pp. 404–426. | ||
+ | |- | ||
+ | |valign="top"|{{Ref|EG}}|| L.C. Evans, R.F. Gariepy, "Measure theory and fine properties of functions" Studies in Advanced Mathematics. CRC Press, Boca Raton, FL, 1992. {{MR|1158660}} {{ZBL|0804.2800}} | ||
+ | |- | ||
+ | |valign="top"|{{Ref|Fe}}|| H. Federer, "Geometric measure theory". Volume 153 of Die Grundlehren der mathematischen Wissenschaften. Springer-Verlag New York Inc., New York, 1969. {{MR|0257325}} {{ZBL|0874.49001}} | ||
+ | |- | ||
+ | |valign="top"|{{Ref|Ha}}|| P.R. Halmos, "Measure theory" , v. Nostrand (1950) {{MR|0033869}} {{ZBL|0040.16802}} | ||
+ | |- | ||
+ | |valign="top"|{{Ref|KP}}|| S. G. Krantz, H. Parks, "Geometric Integration Theory", Birkhäuser (2008). | ||
+ | |- | ||
+ | |valign="top"|{{Ref|Ma}}|| P. Mattila, "Geometry of sets and measures in Euclidean spaces. Fractals and rectifiability". Cambridge Studies in Advanced Mathematics, 44. Cambridge University Press, Cambridge, 1995. {{MR|1333890}} {{ZBL|0911.28005}} | ||
+ | |- | ||
+ | |valign="top"|{{Ref|Mu}}|| M. E. Munroe, "Introduction to Measure and Integration". Addison Wesley (1953). | ||
+ | |- | ||
+ | |valign="top"|{{Ref|Ro}}|| H.L. Royden, "Real analysis" , Macmillan (1969). {{MR|0151555}} {{ZBL|0197.03501}} | ||
+ | |- | ||
+ | |} |
Latest revision as of 09:27, 7 December 2012
2020 Mathematics Subject Classification: Primary: 28A [MSN][ZBL]
The term might refer to different objects in classical measure theory.
Caratheodory measures and outer measures in metric spaces
Consider an outer measure $\mu$ defined on the class $\mathcal{P} (X)$ of subsets of a metric space $(X,d)$. $\mu$ is a Caratheodory outer measure, more often called metric outer measure (cp. with Section 11 of [Ha]), if \begin{equation}\label{e:additive} \mu (A\cup B) = \mu (A) + \mu (B) \end{equation} for every pair of sets $A, B\subset X$ which have positive distance (i.e. such that $\inf \{d(x,y): x\in A, y\in B\} > 0$). A theorem due to Caratheodory shows then that the Borel sets are $\mu$-measurable (see Outer measure#Caratheodory criterion, also for the notion of $\mu$-measurability). The restriction of $\mu$ to the $\sigma$-algebra of $\mu$-measurable sets is called, by some authors, the Caratheodory measure induced by the metric outer measure $\mu$.
The converse is also true: if $\mu$ is an outer measure on a metric space $(X,d)$ for which the open set are $\mu$-measurable, then $\mu$ is a metric outer measure (see for instance Remark (8c) of Section 11 in [Ha]).
Caratheodory outer measures with respect to a class of functions
More generally, given a set $X$ and a class $\Gamma$ of real functions on $X$, some authors (see for instance Section 7 of Chapter 12 in [Ro]) call Caratheodory outer measures with respect to $\Gamma$ those outer measures $\mu$ on $\mathcal{P} (X)$ with the property that \eqref{e:additive} holds when $A$ and $B$ are separated by $\Gamma$, i.e. when there is a function $\varphi\in \Gamma$ with $\inf_A\; \varphi > \sup_B \varphi$ or $\inf_B\;\varphi > \sup_B\; \varphi$.
If $(X,d)$ is a metric space and we chose as $\Gamma$ the set of functions of type $x\mapsto {\rm dist}\, (x, E)$ with $E\subset X$, then a Caratheodory outer measure with respect to $\Gamma$ corresponds to a Caratheodory outer measure in the sense of the previous section.
Caratheodory (outer) measures in the Euclidean space
Some authors use the term Caratheodory (outer) measures for a special class of outer measures defined on the subsets of the euclidean space $\mathbb R^n$ and constructed in a fashion similar to the usual Hausdorff (outer) measures. Cp. for instance with Sections 2.1.3-2.1.4-2.1.5 of [KP] and Sections 2.10.2-2.10.3-2.10.4 of [Fe].
References
[Ca] | C. Carathéodory, "Über das lineare Mass von Punktmengen, eine Verallgemeinerung des Längenbegriffs" Nachr. Gesell. Wiss. Göttingen (1914) pp. 404–426. |
[EG] | L.C. Evans, R.F. Gariepy, "Measure theory and fine properties of functions" Studies in Advanced Mathematics. CRC Press, Boca Raton, FL, 1992. MR1158660 Zbl 0804.2800 |
[Fe] | H. Federer, "Geometric measure theory". Volume 153 of Die Grundlehren der mathematischen Wissenschaften. Springer-Verlag New York Inc., New York, 1969. MR0257325 Zbl 0874.49001 |
[Ha] | P.R. Halmos, "Measure theory" , v. Nostrand (1950) MR0033869 Zbl 0040.16802 |
[KP] | S. G. Krantz, H. Parks, "Geometric Integration Theory", Birkhäuser (2008). |
[Ma] | P. Mattila, "Geometry of sets and measures in Euclidean spaces. Fractals and rectifiability". Cambridge Studies in Advanced Mathematics, 44. Cambridge University Press, Cambridge, 1995. MR1333890 Zbl 0911.28005 |
[Mu] | M. E. Munroe, "Introduction to Measure and Integration". Addison Wesley (1953). |
[Ro] | H.L. Royden, "Real analysis" , Macmillan (1969). MR0151555 Zbl 0197.03501 |
Carathéodory measure. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Carath%C3%A9odory_measure&oldid=23222