Brandt, C.

Feiste, U.

Hasse, H.

Czech digital mathematics library

Measures representable as $p$-dimensional Hausdorff measures

Institute of Mathematics AS CR, v. v. i.

WSAA 8C. Brandt; Uwe Feiste; H. HasseMeasures representable as p-dimensional Hausdorff measuresIn: Zdeněk Frolík (ed.): Abstracta. 8th Winter School on Abstract Analysis.Czechoslovak Academy of Sciences, Praha, 1980. pp. 26--28.Persistent URL: http://dml.cz/dmlcz/701169Terms of use:© Institute of Mathematics of the Academy of Sciences of the Czech Republic,1980Institute of Mathematics of the Academy of Sciences of the Czech Republicprovides access to digitized documents strictly for personal use. Each copy of anypart of this document must contain these Terms of use.This paper has been digitized, optimized for electronic deliveryand stamped with digital signature within the project DML-CZ:The Czech Digital Mathematics Library http://project.dml.czEIGHTH WINTER SCHOOL ON ABSTRACT ANALYSIS (1980) Measures representable as p-dinensional Hausdorff measures C. Bandt , U. Peiste and H. Haase Let (X,r) be a metric space, and let d(A) denote the diame-ter of A . For every positive real number p , a Borel measure m^ on X may be defined by the following formula: mrP(B)-lim i n f { £ d P ( A ± ) | ^ A±2B f d( A i )<t} mpp is called the p-dimensional Hausdorff measure on (X',r). This concept developed by Hausdorff Cll in 1913 has an intuitive geometric meaning. Take p=1 . d(A) might be called the lenght of A (it is the same for an interval on the real line). The 1-dimensional measure of a Borel set B is approximated in the above definition by the sum of the . "lenghts" of small sets which are needed to cover B. To get Lebesgue measure of a set B in the plane we have to cover B by small squares or circles and to add the area of these sets* the area of A now given by d (A) (p=2) • The concept of Hausdorff measure has been neglected in recent time. We emphasize its importance by showing the ' simple but astonishing fact that every locally finite diffuse (i.e. m({x))«0 for all x) measure m on Hn being positive on open sets is an n-dimensional Hausdorff measure with respect to a certain metric compatible with Euclidean topology. Note that every Hausdorff measure is diffuse by definition. . Example Every locally finite diffuse Borel measure m on R is the 1-dimensional Hausdorff measure generated by the pseudometrio r(xfy) » m([xfyf) . If m is positive on all intervals, r is a metric compatible with Euclidean topology. " Pr6of: Since d(A)» m(Cinf A, sup A Q * a(A) f m ^ B ) * m(B) for all B . If B is an interval, there are coverings of B by disjoint intervale of arbitrary small diameter, .m and . mp1 agree on intervals, thus on all Borel sets. % 27 -i * • Remarkt This shows that Hausdorff measures generated by topo-logically equivalent metrics need not be absolutely continuous (there are diffuse locally positive finite Borer measures on R singular to Lebesgue measure). v On the other hand, we can show that every Borel measure m on a locally ̂compact metric space (X,r) given by % m(B)» B / f dm? where f is a positive continuous function, Is the p-dimen-sional Hausdorff measure with respect to a metric rv topolo-gically equivalent to r • r1 Is given by the "line integral11 off,thati 8 ^ ^ ) + f ( j \ / . -•; r'(x,y) * inf i C *—£ i:LL",r(ei»6i+t> 1 0^»°i«iV*| Proposition Let f m be a finite Borel measure on Cantor space D-3̂ 0,1) dr.a locally finite,measure on D-{o}*, and let m(U)>0 for every open U •*Then for,every p>0 , mJLs a p-di-mensional Hausdorff measure with respect to a metric on D , compatible with the product, topology, v Proof: Let r be a metric on D generating the product..topology. J-6* £Q > n=1>2,#*. be a sequence. of partitions of D into clopen subsets, such /that E ^ is & refinement of I> for every n and*that d(U^)<i for U^€.P^ • For two; different * n a n -̂n points x,y of D let n(x,y) be the smallest n for which Pg separates x and y and let \ r1 (x$y)^m(\](xfy)) where U(x,y) is the member of P^A yV-,1*containing x and y • It is easy to see that r' is an ultrametric, that is, it satisfies r'(x,y) < max(r*(x,2), r'(y,z)) for all x,y,z - The topology ' generated by r1 is the product topology since it has the open base consisting of all sets of the £ n • How m̂ JS-ra* follows from di(A)» miri^m(U) | U3A , U« £/Pn} * m(A) • ,For every compact set Band every o>0 there is a neighborhood Uof B with m(U-B)<J* . Then r(B;D-U)«g > 0 .f. and for every n with. i<£ the sets of |L intersecting B form a disjoint covering n -a. t * g A of B with union Smaller than U. Thus m and m_,, agree on. compact sejfs, hence on Borel-sets; Since r* is an ultrame-tric, (r*)? is a metric for every positive p, and .m-srâ , is^ the p-dimensional Hausdorff measure with respect to •. .. (r1)?* >. #Proposition Let 5 be a finite or rf-finite non-atomic measure on a separated, and separable Borel space (X»Jfy and p>0 . Ihere is a*wetric r.on 2* with vm * m^P • ""* * . . ̂  . 28 Proof: All separable separated Borel spaces are isomorphic. There is.a one-to-one mapping f from Z onto D inducing an isomorphism between *A and the Borel -i-algebr% of D . In the i-fihite case f iX^ik-^pl* DxH may be chosen in such a way that msf*5 becomes locally finite* Let r1 be the metric constructed above from the measure ,m=f•£ on D * Then r(xfy)» (rKf<x)tf(y)}>1/P yields,the desired metric on X • Proposition Let m he a locally finite diffuse Borel measure on E n being positive on every open set* Shen m is the n-dimensional Hausdorff measure with respect to a certain metric r* compatible with Euclidean topology* Proof: Oxtoby and Olam tZZ proved that there is a homeomor-phism h from Rn onto R11, for infinite' m and onto l0fk£n for finite, m , such that m(3) equals the Lebesgue measure i(h(B)) for all Borel sets B * Since %* nj* where r de-notes the .maxHtii^ trie on E n we have only to put r'{xfy). r(h(x)fh(y)) . Remark! Obviously, this statement remains valid for diffuse finite Borel measures on (measurable) subsets of H21 • It may also be generalised to manifolds, and with some modification to arbitrary separable metric spaces* Acknowledgement Our lecture on the Winter School did not contain,the last two propositions* The use of isomorphism theorems "Was proposed in discussions with several partici-pants of the conference* We are grateful to professors £. Grsegorek* Z. Lipecki, P* Mankiewicz anii Q.B*F. Thomas who also cbnf Instated us in further working on Hausdorff measures* References • £?3 P. Hausdorff, AuBeres Mafl und Djjnension, Math.Ann* 79 ; ' {1919), 157-179 C23 J«C* Oxtoby and S.lf» Ulam ,. Measure-preserving homeo-. , • morphisms and 'metrical transitivity, Ann* of Math. (2) 42 <1941># 874-920 

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Measures representable as $p$ -dimensional Hausdorff measures

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Measures representable as ppp-dimensional Hausdorff measures

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Measures representable as $p$ -dimensional Hausdorff measures