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Questions about abstract measure and Lebesgue integral theory. Also concerns such properties as measurability of maps and sets.

23 votes
2 answers
7k views

What is a Gaussian measure?

Let $X$ be a topological affine space. A Gaussian measure on $X$ is characterized by the property that its finite-dimensional projections are multivariate Gaussian distributions. Is there a direct ch …
Tom LaGatta's user avatar
  • 8,532
18 votes
3 answers
2k views

How do we express measurable spaces using type theory?

A measurable space $(X,\mathcal X)$ consists of a set $X$ equipped with a $\sigma$-algebra of subsets $\mathcal X$. I would like to write computer programs involving measurable spaces, but to the best …
Tom LaGatta's user avatar
  • 8,532
18 votes
3 answers
1k views

Is there a natural measurable structure on the $\sigma$-algebra of a measurable space?

Let $(X, \Sigma)$ denote a measurable space. Is there a non-trivial $\sigma$-algebra $\Sigma^1$ of subsets of $\Sigma$ so that $(\Sigma, \Sigma^1)$ is also a measurable space? Here is one natural …
Tom LaGatta's user avatar
  • 8,532
17 votes
7 answers
1k views

Examples of toposes for analysts

I've read that toposes are extremely important in modern mathematics, but I find the definitions and examples given on the nLab page a little too abstract to understand. Can you provide some examples …
17 votes
5 answers
3k views

Conditional probabilities are measurable functions - when are they continuous?

Let $\Omega$ be a Banach space; for the sake of this post, we will take $\Omega = {\mathbb R}^2$, but I am more interested in the infinite dimensional setting. Take $\mathcal F$ to be the Borel $\sig …
Tom LaGatta's user avatar
  • 8,532
15 votes
3 answers
2k views

Disintegrations are measurable measures - when are they continuous?

This is a sequel to another question I have asked. The notion of disintegration is a refinement of conditional probability to spaces which have more structure than abstract probability spaces; someti …
Tom LaGatta's user avatar
  • 8,532
12 votes

Proof of Krylov-Bogoliubov theorem

In addition to the excellent answers above, I also suggest the nice survey Oxtoby, Ergodic Sets (Zbl 0046.11504, MR47262, DOI: 10.1090/S0002-9904-1952-09580-X). Introduction. Ergodic sets were introd …
Tom LaGatta's user avatar
  • 8,532
12 votes
3 answers
862 views

Measure theory in nuclear spaces

Much of the literature on measure theory in linear spaces focuses on the case of normed linear spaces (e.g., the outstanding book by Vakhania, or its sequel). However, nuclear linear spaces "as far fr …
Tom LaGatta's user avatar
  • 8,532
12 votes
2 answers
3k views

Does there exist an event independent of a given sigma-algebra?

The following question came up in a discussion with my advisor: Let $(\Omega, \mathcal F, \mathbb P)$ be a non-trivial probability space, and suppose that $\mathcal G$ is a proper sub-$\sigma$-alg …
Tom LaGatta's user avatar
  • 8,532
11 votes
1 answer
2k views

Do Measurable Cardinals Exist? (assuming ZFC)

In Appendix B of his Uniform Central Limit Theorems (1999), Dudley writes: It is consistent with the usual axioms of set theory (including the axiom f choice) that there are no measurable cardinals, …
Tom LaGatta's user avatar
  • 8,532
10 votes
2 answers
2k views

When is a space of measures a measurable space?

Let $X$ denote a measurable space, that is, a set equipped with a $\sigma$-algebra $\Sigma(X)$. Let $M(X)$ denote the space of real-valued measures over $X$. This is a vector space over the real numbe …
Tom LaGatta's user avatar
  • 8,532
9 votes
2 answers
586 views

Fixed objects of the M endofunctor on category Meas

Consider the category $\operatorname{Meas}$ of measurable spaces: its objects are sets equipped with $\sigma$-algebras, and its morphisms are measurable functions between spaces. As Gerald Edgar & M …
Tom LaGatta's user avatar
  • 8,532
8 votes

Is there a natural measures on the space of measurable functions?

Sorry for the necromancy. Here's an attempt at constructing a $\sigma$-algebra using the tensor product of $\sigma$-algebras. This should likely not result in a Borel structure (i.e., a $\sigma$-algeb …
8 votes
4 answers
1k views

Is a measurable homomorphism on a Lie group smooth?

Let $G$ be a Lie group, and let $\mathcal B(G)$ its Borel $\sigma$-algebra. Suppose that $f : G \to G$ is a Borel-measurable homomorphism. Is $f$ smooth? Edit: My original question said "measurable …
Tom LaGatta's user avatar
  • 8,532
7 votes

Can one view the Independent Product in Probability categorially?

One resource you may like is this recent paper by Culbertson and Sturtz on A Categorical Foundation for Bayesian Probability. Here are some thoughts on the category $\mathrm{Meas}$ of measurable sp …

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