Questions related to measures, sigma-algebras, measure spaces, Lebesgue integration and the like.

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1answer
17 views

Mean value formula integrals

Let $f: B(0,R) \rightarrow \mathbb{R}$ be a continuous function. Then I was wondering whether $$\frac{1}{\text{area}(\partial B(0,r))} \int_{\partial B(0,r)} (f(x)-f(0)) dS(x) \rightarrow_{r ...
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0answers
19 views

Equivalent Definition of Weak $L^{p}$ (Quasi-) Norm

For a sigma-finite measure space $(X,\Sigma,\mu)$, the weak $L^p$ (hereafter denoted $L^{p,\infty}$) is defined by $$\|f\|_{L^{p,\infty}}:=\sup_{t>0}t\mu(|f|>t)^{1/p}, \qquad (1\leq ...
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0answers
15 views

Explicit construction of Haar measure on a profinite group

Let $G$ be a profinite group. It is known that in $G$, every neighborhood of the identity element contains an open compact subgroup. I would like to explicitly construct the Haar measure on $G$. The ...
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2answers
39 views

intuition of mass function of random variable

When we are using $P\{X=x\}$ it seems like intuitively there is a function from $T$ (or measure from $\mathcal{B}(T)$) to $[0,1]$. What is the theoretical foundation behind this intuition?
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12 views

What is the interpretation of $\nu(dy - x)$ where $\nu$ is a Lévy measure?

In a paper I am reading, it is seemingly suggested that, if $\nu(dx)$ is a Lévy measure, then the following holds for a function $f(x)$ which is smooth (and satsifies some integrability conditions): ...
2
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1answer
30 views

Looking for a clarification of the Suslin $\mathcal{A}$-Operation with a (finite) example

I have a problem concerning the output of (and the intuition behind) the Suslin $\mathcal{A}$-Operation. More specifically, I really don't see exactly what the output of it really is (even if I can ...
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0answers
23 views

Understanding product $\sigma$-algebra

Let $\{X_\alpha\}_{\alpha \in A}$ be an indexed collection of nonempty sets, $X = \prod _{\alpha \in A}X_\alpha$, and $\pi _\alpha: X \rightarrow X_\alpha$ the coordinate maps. If $M_\alpha$ is a ...
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1answer
18 views

Volume density on a Riemannian manifold as a measure

I am having some trouble in seeing exactly how the Riemannian density form gives rise to a measure on $\text{Borel(M)}$. Let $(M,g)$ be a Riemannian manifold. We have the Riemannian density $\mu_g$. ...
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0answers
13 views

Calculate $\sup_n\left|\int_0^1 \left(1-e^{i\alpha_n x}\right) f(x)d\mu(x)\right|$, where $\mu$ is a singular Borel probability measure on $[0, 1)$.

Let $\mu$ be a singular Borel probability measure on $[0, 1)$, and $f\in L^2(\mu)$. Calculate $$\sup_n\left|\int_0^1 \left(1-e^{i\alpha_n x}\right) f(x)d\mu(x)\right|$$ where $\alpha_n\in\mathbb R$ ...
2
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1answer
31 views

Class of subsets which is not a $\sigma$-ring

I can't find a non-empty class that is closed under countable intersections and symmetric differences, but it's not a $\sigma$-ring. Any ideas?
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1answer
23 views

Is there a Markov-type inequality for the Median?

Markov's theorem states that $P(|X| \geq a) \leq \frac{E[|X|]}{a}$. Is there an similar type of inequality that involves the median (somehow I doub't it, but I make no claim to comprehensive knowledge ...
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1answer
183 views

If the weighted $L^p$ norm of a measurable function is finite, is the weighted $L^p$ norm of the antiderivative also finite?

Let $f \colon \mathbb{R} \rightarrow \mathbb{R}$ be a measurable function such that $$ \int_{-\infty}^{\infty} |f|^p e^{-x^2} \,dx < \infty. $$ Define $g \colon \mathbb{R} \rightarrow ...
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0answers
20 views

Is there a difference between $\mu_1 \times \mu_2$ and $\mu_1 \otimes \mu_2$ in measure theory?

I sometime see the tensor product symbol used when referring to product meauress, but I've also seen the cartesian product symbol used too. Is there a difference. I have had a hard time finding an ...
5
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1answer
179 views

Show that the union over a collection of compact cubes in $\mathbb{R}^n$ is Lebesgue measurable

Let $\mathcal{K}$ be a (not necessarily countable) collection of compact cubes in $\mathbb{R}^n$. Show that $\cup\{K:K\in \mathcal{K}\}$ is a Lebesgue set (Measurable with respect to the Lebesgue ...
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0answers
14 views

Sets cut into two halves of equal size by any straight line through a particular point

Is there an easy characterization of all sets $M \subseteq \mathbb{R}^2$ with the following property? A point $(x_M,y_M)$ (which may depend on $M$) exists such that each straight line through ...
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0answers
73 views

Show that $\overline{D}_{\varepsilon}$ is a connected jordan region in $\mathbb{R}^{n}.$

Definition. Let $E$ be a nonempty subset of $\mathbb{R}^{n}$.The distance from a point $\mathbb{x}\in\mathbb{R}^{n}$ to set $E$ is defined by ...
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1answer
39 views

How to prove the uniqueness of probability measure

Probability essentials P-21 Theorem 4.1 (b) Let $(p_\omega)_{\omega \in \Omega}$ be a family of real numbers indexed by the finite or countable set $\Omega$. Then there exists a unique probability ...
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3answers
66 views

Example of disjoint union of sets which does not have additive measure

I had a question about the additivity property of the outer measure. Can someone provide an example of a disjoint union of sets which doesn't have an outer measure equal to the sum of the outer ...
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0answers
30 views

Premeasure on $\mathcal{A}$ and $\mu^{*}$ proof

This proposition comes from Real Analysis by Folland: Some background information: (1.10) Let $\epsilon\subset P(X)$ and $p:\epsilon\rightarrow [0,\infty]$ be such that $\emptyset\in\epsilon$, ...
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1answer
34 views

Question about formula for total variation of complex measure from Real Analysis of Folland

Let $\nu$ be a complex measure on $(X, \mathcal{M})$. If $E \in \mathcal{M}$, define: $\mu_1(E) = \sup\{\sum_1^n{|v(E_j)|}:n \in N, E_1, ..., E_n$ disjoint$, E = \bigcup_1^n{E_j}\}$ ...
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1answer
18 views

Predictability of $\int^t_0 f(X_s)\,\mathrm ds$ where $X$ is a Lévy process

Let $X_t$ be a Lévy process and $f(x)$ some smooth function. Under what conditions is $$ Y_t = \int^t_0 f(X_s)\,\mathrm ds$$ predictable? Not sure how to investigate this. It is clearly adapted, so ...
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1answer
37 views

Property of a set of a positive Lebesgue measure

I am trying to see whether it is true that in any set of a positive Lebesgue measure in $R^2$ we can always find two points $(a_1,a_2)$ and $(b_1,b_2)$ such that the following hold: $a_1>b_1$ ...
4
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1answer
76 views

Upper Bound for Operator Norm in Marcinkiewicz Interpolation Theorem

Exercise 1.3.3(c) Let $0<p_0<p<p_1<\infty$ and let $T$ be an operator as in Theorem 1.3.2($\|T(f)\|_{L^{p_0,\infty}(Y)}\leq A_0\|f\|_{L^{p_0}(X)}$ for all $f\in L^{p_0}(X)$ and ...
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1answer
53 views

Convergence in $L^{p_1}$ and $L^{p_2}$

Suppose $f_k$ is a sequence of $\mu$-measurable function. Let $p_1$ and $p_2\in[1,\infty)$, and $f_k\in L^{p_1}\cap L^{p_2}$. Also suppose that there exists $g\in L^{p_1}$ and $h\in L^{p_2}$ such that ...
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1answer
107 views

Sufficient Condition for $f\in L^{1}(\mathbb{R}^{d})$ to belong to $L^{2}(\mathbb{R}^{d})$

Question. Let $\left\{\varphi_{j}\right\}$ be a complete orthonormal system for $L^{2}(\mathbb{R}^{d})$ such that each $\varphi_{j}\in C_{b}(\mathbb{R}^{d})$ (the space of continuous, bounded ...
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1answer
36 views

Repeated extension of Lebesgue measure

In Halmos' Measure Theory, section 16, exercise 2 deals with the extension of a $\sigma$-finite measure $\mu$ defined on a $\sigma$-ring $S$ to any set $M$ in the hereditary $\sigma$-ring induced by ...
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0answers
52 views

Closeness of measures on a cardinal

Given an uncountable $\kappa$ and a $\kappa$-complete nontrivial non-normal ultrafilter on $\kappa$, and some $g:\kappa\to\kappa$ with $<_{U}$-rank $\kappa$ (where $f_0<_Uf_1$ iff ...
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1answer
39 views

Jensen's inequality problem [on hold]

I want to know an example of a infinite measure space $(\Omega, \mathcal{F},\mu)$, real valued function $g$ and convex function $\phi$ defined on the real line s.t. $$\phi\left(\int g d\mu\right) > ...
5
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2answers
82 views

for each $\epsilon >0$ there is a $\delta >0$ such that whenever $m(A)<\delta$, $\int_A f(x)dx <\epsilon$

This is an old preliminary exam problem: Show that, for every nonnegative Lebesgue integrable function $f:[0,1]\rightarrow \mathbb{R}$ and every $\epsilon>0$ there exists a $\delta>0$ such ...
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2answers
41 views

$\sigma$-algebra of $\mathbb{R}$ generated by $\mathcal{P}(\mathbb{N})$

What is the $\sigma$-algebra of $\mathbb{R}$ generated by $\mathcal{P}(\mathbb{N})$? I thought it is $$\Sigma = \{\emptyset, \mathbb{N}, \mathcal{P}(\mathbb{N}), \mathbb{R}, \mathbb{R}-\mathbb{N}, ...
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0answers
36 views

Generating structure of Borel field

On P.32 of [P.Billingsley] Probability and Measure, 3ed, 1993, the author wrote: ...and there are Borel sets that cannot be arrived at from the intervals by any finite sequence of set-theoretic ...
2
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0answers
23 views

Example of a bounded simple process $A_t$ that changes value only once s.t. $\int_0^t A_s dB_s$ doesn't have normal distribution?

As the title of the question suggests, what is an example of a bounded simple process $A_t$ that changes value only once such that$$\int_0^t A_s\,dB_s$$does not have a normal distribution?
2
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1answer
33 views

Measurability of marginal distributions of a random measurable function

For a probability space $(\Omega, \mathcal F, \mathsf P)$, let $X \colon \Omega \times [0,1] \to \mathbf R \colon (\omega, t) \mapsto X(\omega,t)$ be a random Borel function (i.e. an $(\mathcal ...
1
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1answer
30 views

Norm triangle inequality for convolutions proof

I'm trying to prove that $$\|f*g\|_{L_1}\le{\|f\|_{L_1}\|g\|_{L_1}}$$ with respect to a Haar measure over a group G. Using Fubini's theorem, I'm up to ...
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1answer
24 views

convolution of probablity measures

What do we mean by convolution of measures? With example What is the difference between convolution of measures and convolution of functions? What is probability measure? Give an example of ...
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1answer
40 views

Help with a Lebesgue integration problem.

The question is the following, Let $f:\Bbb{R}\rightarrow \Bbb{R}$ be a Lebesgue integrable function. Show that $\mathop {\lim }\limits_{t \to \infty } \int_\Bbb{R} {f(x)\cos (xt)dx} = \mathop ...
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1answer
28 views

Borel measure induced by the Cantor function?

In an example to measure being mutally singular, the book has an example I do not understand. First the book has the definition: Mutually Singular Measure Let $(\Omega,\mathcal{A})$ be a ...
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1answer
33 views

Properties of decreasing sequence of Lebesgue measurable sets.

I'm trying to prove a property of Lebesgue measure sets that says: If the $A_{k}$'s are measurable and $A_{1} \supset A_{2} \supset A_{3} \supset \ldots,$ and if $\lambda (A_{1}) < \infty, $ then ...
2
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1answer
19 views

Help with a sigma-algebra problem with random variables (show $\sigma(X_S)\subseteq \sigma(X_T)$ if $S\subseteq T$)

My problem is as follows: Let $X_S$ and $X_T$ be two stochastic processes where $S,T$ are index sets. Let $\sigma(X_S)$ and $\sigma(X_T)$ denote the sigma-algebra generated by $X_S$ and $X_T$. ...
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61 views

Lebesgue Premeasure via Transfinite Induction

If $I=[a,b)$ we write $|I|=b-a$ for the length of $I$. Given a theorem of Caratheodory, the tricky part in showing the existence of Lebesgue measure is this: Lemma If $[0,1)$ is the disjoint union of ...
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2answers
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If $f_k \to f$ a.e. and the $L^p$ norms converge, then $f_k \to f$ in $L^p$

Let $1\leq p < \infty$. Suppose that $\{f_k\} \subset L^p$ (the domain here does not necessarily have to be finite), $f_k \to f$ almost everywhere, and $\|f_k\|_{L^p} \to \|f\|_{L^p}$. Why is it ...
2
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0answers
39 views

Example of Measure of non-compactness?

I can't understand the following example of measure of non-compactness, which was given in a research article. Definition: A nonnegative function $\phi$ defined on the bounded subsets of $X$ will ...
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1answer
50 views

Easy proof for existence of Lebesgue-premeasure

In the lecture on measure theory I attended last semester, we had a sort of complicated technical proof for the existence of the Lebesgue-premeasure. However, I can't see why this easier argument does ...
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2answers
69 views

Proving a set is Lebesgue Measurable [duplicate]

Measure is a serious weak point of mine, and I cannot figure out this problem: Let $E \subset \mathbb{R}$ be Lebesgue measurable. Suppose that for all open intervals $I$, we have $m(E\cap I) \leq ...
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1answer
317 views

Analysis - Fourier Transforms - show that convolution of characteristic functions is continuous

I would appreciate any instruction on the following exercise from real and complex analysis: Suppose $A$ and $B$ are measurable subsets of $\Re^1$, having finite positive measure. Show that the ...
2
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1answer
42 views

Lebsegue measure of $\{ 0<x \leq 1: x \sin \left(\frac{\pi}{2x}\right) \geq 0 \}$

Find the Lebsegue measure of the set $A= \left\{ 0<x \leq 1: x \sin \left(\frac{\pi}{2x}\right) \geq 0 \right\}$. The answer given is $1 - \ln \sqrt{2}$. My thought: I only know that Lebsegue ...
3
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1answer
38 views

Show that $\mu(f)\mu(1/f)\geq\mu(\Omega)^2$

Prove that $\mu(\Omega)^2\leq\int f \,d\mu\int\frac{1}{f}\,d\mu$. I don't know if that what I did is correct or if it will help to solve the problem, but here it is: Using the Hölder inequality ...
4
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2answers
55 views

If $\mu(|f_n|^p)$ is bounded and $f_n\to f$ in measure then $f_n\to f$ in $L^1$

Let $(f_n)_{n\in\mathbb{N}}$ be a sequence of real measurable functions s.t., (a) The sequence $\displaystyle(\int |f_n|^p\ \mathsf d\mu)_{n\in\Bbb{N}}$ is bounded. (b) The sequence ...
1
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1answer
34 views

Help verify a solution showing $f\left(x \right)=\int_\Bbb{R} {{\chi _A}\left(y \right){\chi _B}\left( {x-y} \right)dy} $ is well-defined everywhere

The question is, Let $A,B⊂[0,1]$ be measurable sets with $|A|>1/2$,$|B|>1/2$ where $|*|$ denotes Lebesgue measure. Prove that a. $|A⋂(1-B)|>0$ where $1-B≔{1-x:x∈B}$ and conclude that ...
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0answers
15 views

What is the difference between a “Borel probability measure” and a “singular Borel probability measure”? [on hold]

What is the difference between a "Borel probability measure" and a "singular Borel probability measure"? When a probability meausure is said singular? Thanks in advance.