proof that the outer (Lebesgue) measure of an interval is its length
We begin with the case in which we have a bounded interval, say [a,b]. Since the open interval (a-ε,b+ε)
contains [a,b] for each positive number ε, we have
m*[a,b]≤b-a+2ε. But since this is true for each positive ε, we must have m*[a,b]≤b-a. Thus we only have to show that m*[a,b]≥b-a; for this it suffices to show that if {In} is a countable
open cover by intervals of [a,b], then
∑l(In)≥b-a. |
By the Heine-Borel theorem, any collection of open intervals [a,b] contains a finite subcollection that also cover [a,b] and since the sum of the lengths of the finite subcollection is no greater than the sum of the original one, it suffices to prove the inequality
for finite collections {In} that cover [a,b]. Since a is contained in ⋃In, there must be one of the In’s that contains a. Let this be the interval (a1,b1). We then have a1<a<b1. If b1≤b, then b1∈[a,b], and since b1∉(a1,b1), there must be an interval (a2,b2) in the collection {In} such that b1∈(a2,b2), that is a2<b1<b2. Continuing in this fashion, we obtain a sequence (a1,b1),…,(ak,bk) from the collection {In} such that ai<bi-1<bi. Since {In} is a finite collection our process must terminate with some interval (ak,bk). But it terminates only if b∈(ak,bk), that is if ak<b<bk. Thus
∑l(In) | ≥∑l(ai,bi) | ||
=(bk-ak)+(bk-1-ak-1)+…+(b1-a1) | |||
=bk-(ak-bk-1)-(ak-1-bk-2)-…-(a2-b1)-a1 | |||
>bk-a1, |
since ai<bi-1. But bk>b and a1<a and so we have bk-a1>b-a, whence ∑l(In)>b-a. This shows that m*[a,b]=b-a.
If I is any finite interval, then given ε>0, there is a closed intervalJ⊂I such that l(J)>l(I)-ε. Hence
l(I)-ε<l(J)=m*J≤m*I≤m*ˉI=l(ˉI)=l(I), |
where by ˉI we the topological closure of I. Thus for each ε>0, we have l(I)-ε<m*I≤l(I), and so m*I=l(I).
If now I is an unbounded interval, then given any real number Δ, there is a closed interval J⊂I with l(J)=Δ. Hence m*I≥m*J=l(J)=Δ. Since m*I≥Δ for each Δ, it follows m*I=∞=l(I).
References
Royden, H. L. Real analysis. Third edition. Macmillan Publishing Company, New York, 1988.
Title | proof that the outer (Lebesgue) measure of an interval is its length |
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Canonical name | ProofThatTheOuterLebesgueMeasureOfAnIntervalIsItsLength |
Date of creation | 2013-03-22 14:47:04 |
Last modified on | 2013-03-22 14:47:04 |
Owner | Simone (5904) |
Last modified by | Simone (5904) |
Numerical id | 6 |
Author | Simone (5904) |
Entry type | Proof |
Classification | msc 28A12 |
Related topic | LebesgueOuterMeasure |