请输入您要查询的字词:

 

单词 PropertiesForMeasure
释义

properties for measure


Theorem [1, 2, 3, 4]Let (E,,μ) be a measure spaceMathworldPlanetmath, i.e.,let E be a set, let be a σ-algebra of setsMathworldPlanetmathin E, and let μ be a measureMathworldPlanetmath on .Then the following properties hold:

  1. 1.

    Monotonicity: If A,B, and AB, then μ(A)μ(B).

  2. 2.

    If A,B in , AB, and μ(A)<, then

    μ(BA)=μ(B)-μ(A).
  3. 3.

    For any A,B in , we have

    μ(AB)+μ(AB)=μ(A)+μ(B).
  4. 4.

    Subadditivity: If {Ai}i=1 is a collectionMathworldPlanetmath of sets from , then

    μ(i=1Ai)i=1μ(Ai).
  5. 5.

    Continuity from below:If {Ai}i=1 is a collection of sets from such thatAiAi+1 for all i, then

    μ(i=1Ai)=limiμ(Ai).
  6. 6.

    Continuity from above:If {Ai}i=1 is a collection of sets from such thatμ(A1)<, and AiAi+1 for all i, then

    μ(i=1Ai)=limiμ(Ai).

Remarks In (2), the assumptionPlanetmathPlanetmath μ(A)< assuresthat the right hand side is always well defined, i.e., not ofthe form -. Without the assumption we can prove thatμ(B)=μ(A)+μ(BA) (see below).In (3), it is tempting tomove the term μ(AB) to the other side for aesthetic reasons.However, this is only possible if the term is finite.

Proof. For (1), suppose AB. We can thenwrite B as the disjoint unionMathworldPlanetmathPlanetmath B=A(BA), whence

μ(B)=μ(A(BA))=μ(A)+μ(BA).

Since μ(BA)0, the claim follows.Property (2) follows from the above equation; sinceμ(A)<, we can subtract this quantity from both sides.For property (3), we can writeAB=A(BA), whence

μ(AB)=μ(A)+μ(BA)
μ(A)+μ(B).

If μ(AB) is infiniteMathworldPlanetmathPlanetmath, the last inequalityMathworldPlanetmath mustbe equality, and either of μ(A) or μ(B) must be infinite.Together with (1), we obtain that if any of the quantitiesμ(A),μ(B),μ(AB) or μ(AB) is infinite,both sides in the equation are infinite and the claim holds.We can thereforewithout loss of generality assume that all quantities are finite.From AB=B(AB), we have

μ(AB)=μ(B)+μ(AB)

and thus

2μ(AB)=μ(A)+μ(B)+μ(AB)+μ(BA).

For the last two terms we have

μ(AB)+μ(BA)=μ((AB)(BA))
=μ((AB)(AB))
=μ(AB)-μ(AB),

where, in the second equality we have used properties for thesymmetric set difference (http://planetmath.org/SymmetricDifference), andthe last equality follows fromproperty (2). This completesPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath the proof ofproperty (3).For property (4), let us define the sequencePlanetmathPlanetmath {Di}i=1 as

D1=A1,Di=Aik=1i-1Ak.

Now DiDj= for i<j, so {Di} is a sequence ofdisjoint sets.Since i=1Di=i=1Ai, and sinceDiAi, we have

μ(i=1Ai)=μ(i=1Di)
=i=1μ(Di)
i=1μ(Ai),

and property (4) follows.

TODO: proofs for (5)-(6).

References

  • 1 G.B. Folland, Real Analysis: Modern Techniques and Their Applications, 2nd ed, John Wiley & Sons, Inc., 1999.
  • 2 A. Mukherjea, K. Pothoven,Real and Functional analysis,Plenum press, 1978.
  • 3 D.L. Cohn, Measure Theory, Birkhäuser, 1980.
  • 4 A. Friedman,Foundations of Modern AnalysisMathworldPlanetmath,Dover publications, 1982.
随便看

 

数学辞典收录了18232条数学词条,基本涵盖了常用数学知识及数学英语单词词组的翻译及用法,是数学学习的有利工具。

 

Copyright © 2000-2023 Newdu.com.com All Rights Reserved
更新时间:2025/5/5 2:58:34