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Sequential suprema, infima, limsup, liminf, and pointwise limits of measurable functions are measurable
Statement
Let be a measurable space and let be measurable for every . Then the functions
are measurable. The set
is measurable. In particular, if pointwise, then is measurable.
Facts & Assumptions
Given: A measurable space and measurable functions for .
Threshold measurability characterizes extended-real measurability. (Threshold characterisations of real-valued and extended-real-valued measurability)
For each , the limsup and liminf of the sequence satisfy
and the pointwise limit exists exactly when the limsup and liminf are equal. (Limit superior and limit inferior of a real sequence as and in , A real sequence converges to iff , and diverges to iff both equal )
Proof
Let and . Then for every real [L1, given] ,
Since each threshold set on the right is measurable, [L1] gives measurability of and . [L1, given]
For each , the tail functions [step 1.1, L2] and are measurable by step 1.1. Applying step 1.1 again to the sequences and and then using [L2] yields measurability of and .
Let and . The equality set [step 2.1, L1, L2] is measurable because
If or , a rational strictly between them separates the two sides; if , every rational lies on the same side of both values. So [L2] makes the pointwise-convergence set measurable. [step 2.1, L1, L2]
If pointwise, then [L2] gives [step 2.1, step 3.1, L2] . Since step 2.1 has already proved that both limiting functions are measurable, is measurable.
Depends on
- Limit superior and limit inferior of a real sequence as $\inf_n \sup_{k \ge n} x_k$ and $\sup_n \inf_{k \ge n} x_k$ in $\overline{\mathbb{R}}$
- A real sequence converges to $L \in \mathbb{R}$ iff $\liminf x_k = \limsup x_k = L$, and diverges to $\pm\infty$ iff both equal $\pm\infty$
- Threshold characterisations of real-valued and extended-real-valued measurability
Used by
- Almost-sure convergence of a random series Definition
- Conditional expectation for nonnegative variables Definition
- Measurable and decomposable operator fields Definition
- The weighted maximal function of a doubling weight Definition
- Upcrossing number of an interval Definition
- Wiener measure on continuous path space Definition
- A deterministic integral construction of a Gaussian process Example
- A Sierpinski gasket computed by hand Example
- A topological invariant mean yields norm-approximately invariant densities Lemma
- Adapted continuous processes are progressively measurable Lemma
- Borel sigma-algebra of continuous path space is generated by coordinates Lemma
- Cauchy sequences in probability have a measurable limit Lemma
- Hilbert cube has a bimeasurable real coding Lemma
- L² convolution on a compact group is Hilbert–Schmidt Lemma
- Measurable Gram-Schmidt and constant-field trivializations on dimension strata Lemma
- Measurable sections have measurable pointwise inner products Lemma
- Rational conditional distribution functions produce real regular kernels Lemma
- Regular conditional kernels factor through a standard borel conditioning variable Lemma
- Reiter functions can be cut down to Følner sets Lemma
- Simultaneous rational conditional distribution function versions Lemma
- Digit-position density determines Hausdorff dimension Proposition
- For sigma-finite measures, the section-measure functions are measurable Proposition
- A function measurable for a completion is almost everywhere equal to one measurable for the original sigma-algebra Theorem
- Birkhoff's theorem for an ergodic probability system Theorem
- Bochner dominated convergence theorem Theorem
- Bochner integrability criterion Theorem
- Cauchy sequences in measure converge in measure Theorem
- Chain rule for globally Lipschitz scalar maps of Sobolev functions Theorem
- Conditional fatou and dominated convergence Theorem
- Direct integrals of measurable Hilbert fields are Hilbert spaces Theorem
- Disintegration of a joint law on standard borel spaces Theorem
- L two kernels give Hilbert–Schmidt operators Theorem
- Levy continuity theorem converse Theorem
- Localized Ito integral Theorem
- Reverse martingale convergence Theorem
- Riesz-Fischer completeness of Lᵖ for 1 ≤ p ≤ ∞ Theorem
- Separable dual spaces have the Radon--Nikodym property Theorem
- Strong Markov property of Brownian motion Theorem
- Taking out what is known Theorem
- The ACL characterisation of W^1,p Theorem
…and 2 more results.
Dependency tree · two levels
27 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Sheldon Axler, Measure, Integration and Real Analysis, Proposition 2.53 (standard reference, not scraped)