properties for measure
Theorem [1, 2, 3, 4]Let be a measure space, i.e.,let be a set, let be a -algebra of sets
in , and let be a measure
on .Then the following properties hold:
- 1.
Monotonicity: If , and , then .
- 2.
If in , , and , then
- 3.
For any in , we have
- 4.
Subadditivity: If is a collection
of sets from , then
- 5.
Continuity from below:If is a collection of sets from such that for all , then
- 6.
Continuity from above:If is a collection of sets from such that, and for all , then
Remarks In (2), the assumption assuresthat the right hand side is always well defined, i.e., not ofthe form . Without the assumption we can prove that (see below).In (3), it is tempting tomove the term to the other side for aesthetic reasons.However, this is only possible if the term is finite.
Proof. For (1), suppose . We can thenwrite as the disjoint union , whence
Since , the claim follows.Property (2) follows from the above equation; since, we can subtract this quantity from both sides.For property (3), we can write, whence
If is infinite, the last inequality
mustbe equality, and either of or must be infinite.Together with (1), we obtain that if any of the quantities or 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 , we have
and thus
For the last two terms we have
where, in the second equality we have used properties for thesymmetric set difference (http://planetmath.org/SymmetricDifference), andthe last equality follows fromproperty (2). This completes the proof ofproperty (3).For property (4), let us define the sequence
as
Now for , so is a sequence ofdisjoint sets.Since , and since, we have
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 Analysis
,Dover publications, 1982.