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

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0
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0answers
10 views

Showing that $\mathcal{A}$ being countable $\Rightarrow f(\mathcal{A})$ is countable - (Algebras/Sigma Algebras)

For the first question my idea was to show that $\sigma(f(\mathcal{A})) \subseteq \sigma(\mathcal{A})$ and $\sigma(\mathcal{A}) \subseteq \sigma(f(\mathcal{A}))$. As for the second question I am at ...
-1
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1answer
36 views

Showing that if $A_{1},A_{2},…$ are all algebras then the union of all of them is an algebra [duplicate]

I am not sure how to show this. It seems obvious but maybe its not. The help would be greatly appreciated!
2
votes
1answer
20 views

Complex Measures: Absolute Continuity [on hold]

Note: This is a lemma for: Spectral Measures: Riemann-Lebesgue Given a positive measure: $$\lambda:\mathcal{A}\to[0,\infty]$$ Consider a complex measure: $$\mu:\mathcal{A}\to\mathbb{C}$$ How to ...
3
votes
2answers
25 views

existence of a borel probability measure on $[0,1]$ such that $\int f d\mu=\lim_{k\to\infty}\frac {1} {N_k} \sum_{i=1}^{N_k}f(x_i)$ given sequence

Hi I'm really suck with this one, i would really appreciate it if any one can help me with this! prove that for $\{x_i\}\subseteq[0,1]$ there are $1\le N_1<N_2...$ and a probability measure $\mu$ ...
1
vote
0answers
14 views

Abstract question in measure theory related to product measure

First of all let me write up some definitions here: (M1),(M2),(M3) are the properties of a measure. (M4) is finite additivity. Hey guys I am having hard time wrapping my head around this question ...
0
votes
1answer
12 views

Is this measure finite, $\sigma$-finite, or a probability measure?

I was a little unsure on this problem. I do have some ideas though. The way I thought of translation invariant is that you can take an interval and shift it, and in the process is will still be the ...
0
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0answers
9 views

Showing Brownian motion is measuable

How can I prove Brownian motion is measurable with respect to the corresponding product sigma algebra? I am struggling to extend the measurability from holding for rational times to all times using ...
1
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3answers
39 views

Union of two $\sigma$-algebras is not $\sigma$-algebra

Here is another very basic analysis problem but that puzzles me: Find an example of set $X$ and its two $\sigma$-algebras $\mathscr A_1$ and $\mathscr A_2$, such that $\mathscr A_1 \cup \mathscr ...
11
votes
1answer
73 views

Measurability Question?

I assigned the following to a class I'm teaching and, to my embarrassment, I cannot come up with a solution. Let $(X,\mathcal B)$ be a measurable space and let $(f_n)_{n\ge 1}$ be a sequence of ...
1
vote
1answer
10 views

A set with non-$\sigma$-algebra monotone class

Working on this very basic analysis problem: Find an example of set $X$ with its monotone class $\mathscr M$ such that $\emptyset, X \in \mathscr M$, but it is not a $\sigma$-algebra. My ...
-1
votes
0answers
12 views

$C(K)^*$ when K is a countable, compact metric space. [on hold]

If K is a countable, compact metric space, then why $C(K)^*$ consists of only purely atomic measures? Also, why $C(K)^*$ is isometric to $\ell_1$ ? (See Topics in Banach space theory by Albiac and ...
1
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1answer
26 views

Nonmeasureable subset of ${\mathbb{R}}^2$ such that no three points are collinear?

I'm exploring the properties of sets in the plane that do not contain a set of three collinear points. In particular, I'm interested in the "largest" they can be. Things I know so far: Assuming the ...
0
votes
0answers
25 views

$\sigma$-algebra generated by a topology [on hold]

Suppose that $(X, \mathcal{T})$ is a topological space. My hunch is that the smallest $\sigma$-algebra on $X$ containing $\mathcal{T}$ is the collection of Borel sets obtained from $\mathcal{T}$. Is ...
0
votes
0answers
26 views

The largest $\sigma$-algebra generated by a subset

It is always possible to find a smallest $\sigma$-algebra that contains any subset $A$ of a given set $X$ (it is, by definition, the intersection of all the $\sigma$-algebras on $X$ that contain $A$). ...
1
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1answer
26 views

Showing that a collection of intervals (see problem) generates the Borel sigma algebra on $(0,1]$

I would be very appreciative if someone could show me how to do this problem so that I can try to get a better understanding of what a Borel sigma algebra is. Examples are how I learn best so seeing ...
1
vote
1answer
21 views

How does a change of measure affect covariance?

Suppose I have the three random variables $X,Y,M$ where $E[M] = 1$ under the measure $P$. Now, suppose I define a new measure $\widetilde P$ so that $\widetilde E[X] = E[M X]$ and $\widetilde E[Y] = ...
0
votes
1answer
18 views

How would I show that $m(\cup_{n \in \mathbb{N}}A_{n}) = \displaystyle\sum_{n \in \mathbb{N}}m(A_{n})$?

Suppose we have that $A_{1}, A_{2}, A_{3}, ...$ is a countable collection of elements of an algebra $\mathcal{A}$. Suppose also that $m$ is a probability measure on $\mathcal{A}$ where $m(A_{i} \cap ...
3
votes
1answer
28 views

Prove or disprove $\nu(E)=\lambda(f(E))$ is a measure provided that $f$ is nondecreasing and satisfies the N-condition.

Suppose $f$ is a non-decreasing continuous function from $[a,b]$ to $\mathbb{R}$, and $\lambda$ is the Lebesgue measure in $\mathbb{R^1}$. Also, $f$ satisfies the property that $f$ maps Lebesgue ...
2
votes
1answer
33 views

$f\in L^p(X,\mu)$ , $f-1\in L^q(X,\mu)$ then $\mu(X) < \infty $

Can some one give a hint how to start to solve : Assume $ 1 \le p,q < \infty $ and $$f\in L^p(X,\mu)$$ now if we assume $$f-1\in L^q(X,\mu)$$ then we have $$\mu (X) < \infty $$ Thanks If ...
2
votes
0answers
18 views

Convergence of Uniformly Distributed Random Variables (n-dimensional)

Suppose that ${U_n} = ({U_{n1}},{U_{n2}},...,{U_{nn}})$ is uniformly distributed over the n-dimensional cube ${C_n}={[0,2]^n}$ for each $n=1,2,...$ That is, that the distribution of ${U_n}$ is ...
0
votes
2answers
32 views

Does the following set contain the Borel $\sigma$ algebra?

Suppose I have $X = \{(2^{-n-1},2^{-n}]:n \in \mathbb{N} \cup \{0\}\}$ and $ K = \sigma(X)$ where $\sigma(X)$ is a sigma algebra. My question is does $K$ contain the Borel $\sigma$-algebra?
0
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1answer
9 views

Manipulating the definition of $\sigma$-algebra generated by a family of sets

I manipulated the standard definition of $\sigma$ algebra generated by a family of sets $\mathcal{A}$, $$ \sigma ( \mathcal{A}) := \bigcap \{ \Sigma \ | \ \Sigma \text{ is a $\sigma$-algebra on } X, ...
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votes
0answers
54 views

A Classification problem in measure theory [on hold]

Let $m$ be the Lebesgue measure on $\Bbb R$. I want to find all measurable functions $f:[0,1] \rightarrow [0,1]$ with the following property. $$\forall A\subseteq [0,1]~ (\text{Lebesgue ...
1
vote
0answers
18 views

Sigma-additive measures on finite discrete spaces

Let $\mathcal{A}$ be a finite algebra of sets, and $\mu$ a $\sigma$-additive function defined on $\mathcal{A}$. Then, the Hahn-Kolmogorov theorem tells us, amongst other things, that $\mu$ can be ...
1
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0answers
6 views

what is the difference between renormalized solution and entropy solutions for nonlinear elliptic PDE?

I want to know the difference between renormalized solution and entropy solutions for nonlinear elliptic PDE when right hand side of the elliptic operator is $L^1.$ Also how does right hand side with ...
0
votes
1answer
19 views

Generating Borel algebra - proof

In the second paragraph of the proof it says: "To prove that $\mathcal{B}(\mathbb{R})$ is also generated by the other classes of intervals, it suffices to prove that any interval $]a,b[$ is contained ...
-3
votes
1answer
43 views

reducible measure functions [on hold]

Let $f:\Bbb R^n \rightarrow \Bbb R^n$ be a nonexpansive map, that is $\|f(x) - f(y)\| \leq \|x-y\|.$ I want to know that if $f$ reduces measure, i.e. if $A,f(A) \subseteq \Bbb R^n $ are measurable, ...
1
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2answers
47 views

Measure Theory Book for My Background / Need

My current Math background is as follows: 1) Read first 7 chapters of Rudin "Principles of Mathematical Analysis" and solved a lot of the given problems. 2) Completed Munkres "Analysis on Manifolds" ...
0
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0answers
19 views

Measure in a locally compact Hausdorff space

Suppose that $X$ is a locally compact Hausdorff space in which every open set is $\sigma$-compact and such that for every compact subset $K$ of $X$ we have $\mu(K) < \infty$. What can be deduced ...
-1
votes
1answer
37 views

$\sigma$-algebra generated by all supersets of $[0, 1]$ [on hold]

I am trying to find the $\sigma$-algebra generated by all subsets $B$ of $\mathbb{R}$ that contain the set $[0, 1]$, i.e., $[0, 1] \subseteq B \subseteq \mathbb{R}$. Any help would be appreciated.
7
votes
1answer
366 views

Is any subset of the Cantor set a Borel set?

As far as I know, the Cantor set is a Borel set because it is the union of a countable collection of closed sets. Now is any subset of the Cantor set a Borel set?
2
votes
0answers
30 views

$\sigma$-algebra $\mathcal{G} \subset \mathcal{F}$ is countably generated iff there is a random variable $X$ such that $\mathcal{G} = \sigma(X)$.

Where can I find a reference to the proof of the fact that a $\sigma$-algebra $\mathcal{G} \subset \mathcal{F}$ is countably generated if and only if there is a random variable $X$ such that ...
1
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0answers
17 views

Clarification on product space, conditional probability measures

Suppose we have two random variables $X_i$ on the probability space $(\Omega_i, \sigma(\Omega_i),P_i)\; i = 1,2$. Now, $P_i$ is just a technical tool and we consider directly the distribution of $X_i$ ...
0
votes
0answers
20 views

If the support of a function is contained in a Borel set, is the support of its derivative also contained there?

Let $f$ be a function such as $\operatorname{supp}(f)\subset Q$ where $Q$ belongs to the Borel $\sigma$-algebra on $\mathbb{R}^d$ Do we have $\operatorname{supp}(f')\subset Q$?
2
votes
3answers
37 views

If $\mu(E)=0$, show that $\mu(E\cup A)=\mu(A\setminus E)=\mu(A)$.

If $\mu(E)=0$, show that $\mu(E\cup A)=\mu(A\setminus E)=\mu(A)$. I just started learning about measure this week, so I don't know any theory about measure except the definition of outer measure ...
0
votes
1answer
17 views

$\lim_{n \to \infty} \int_{\Omega}X_n d \mu = +\infty$ under some conditions

Suppose $X_n$ are measurable functions in $L^1$ defined on the measure space $(\Omega, \mathfrak{F}, \mu)$. Suppose that $0 \leq X_n$ a.e. for all $n$ and $X_n \leq X_{n+1}$ a.e. for all $n$. Thus ...
0
votes
0answers
22 views

Variants of the change-of-variables formula

Consider the following change of variables formula for $f:X\rightarrow Y$, that holds for any "reasonable" $g:B\subseteq Y \rightarrow \mathbb{R}$ and $A\subseteq X$ $$ \int_B g(x)\ {\rm ...
2
votes
1answer
51 views

right continuous representant of L^1 function

Would someone know if a function in $L^1(\Omega)$ or more generally in $L^p(\Omega)$ must have a right continuous representant. $\Omega$ being a open set of $\mathbb{R}$. Thankfully,
1
vote
2answers
49 views

Show that $(\mathcal{M},d)$ is complete metric space

Let $(\Bbb{R},\mathcal{M},\mu)$ be the Lebesgue measure space modulo the equivalence relation $A\sim B$ if $\mu(A\bigtriangleup B)=0$. Let $d(A,B)=\mu(A\bigtriangleup B)$. Show that $(\mathcal{M},d)$ ...
2
votes
1answer
39 views

Struggling with something 4 sources use “clearly” with (involving $\inf$ of a set)

This is technically measure theory but it as much real analysis. We have a Measure $\mu:R\rightarrow\mathbb{R}_{\ge0}\cup\{\infty\}$ We define the outer measure $\mu^*$ to be: ...
2
votes
1answer
38 views

Convergence in measure - product

I'm trying to prove the following statements in Folland's book. Let $(X,\mathcal{M},\mu)$ be a measure space. If $f_n\to f$ in measure and $g_n\to g$ in measure, then $f_n+g_n\to f+g$ in measure and ...
4
votes
1answer
45 views

Calculation of Radon–Nikodym derivative

Suppose the function $X \colon \mathbb{R} \longrightarrow \mathbb{R} \colon x \longmapsto X(x) : = x^2$. I want to calculate the Radon–Nikodym derivative $\frac{\text{d}\lambda_X}{\text{d}\lambda}$, ...
1
vote
0answers
17 views

image or projection of a universally measurable set

Suppose $A \subset [0,1]\times[0,1]$ is universally measurable. Is it true that its projection to the first coordinate is a universally measurable subset of $[0,1]$? What is known is that the ...
3
votes
1answer
38 views

Prove that measure of $A$ is $1$

Let $A\subset (0,1)$ be a Lebesgue measurable set and $\lambda>0$. Suppose that if $0\le a<b\le 1$ then $\mu(A\cap (a,b))\ge \lambda(b-a)$. Prove that $\mu(A)=1$. It is clear that $\lambda \le ...
0
votes
0answers
20 views

Lebesgue Measure of a set satisfying infinitely many solutions of this inequality

I am trying to find the following. Suppose that $\alpha_k > 0$, and $\sum \alpha_k < 0$. Let's consider the set $$A = \{x\in(0,1) | \hbox{the inequality} |x -{p \over q}| < {\alpha_q \over ...
1
vote
1answer
17 views

Complex Borel Measure Decomposes as Discrete + Continuous Measures?

I am trying to prove that any complex Borel measure $\mu$ on a measure space $(X, \mathfrak{M})$ decomposes into the sum of a discrete measure $\lambda$ and a continuous measure $\nu$, $\mu = \lambda ...
3
votes
0answers
25 views

Existence of regular conditional distribution of random variable given the value of another variable

Let $(\Omega, \mathcal{A}, \mathbf{P})$ be a probability space with a measurable function $Y: (\Omega, \mathcal{A}) \rightarrow (E, \mathcal{E})$ and another measurable function $X: (\Omega, ...
0
votes
1answer
12 views

Is the product measure space generated by the filtration adapted to the projection maps?

Let $(\Omega, \mathcal A)$ be a measure space. Consider the product measure space $(\Omega^{\mathbb N}, \mathcal A^{\mathbb N})$ and denote by $\pi_n : \Omega^{\mathbb N} \to \Omega$ the $n$-th ...
1
vote
3answers
35 views

Let $S = \{(a,b] \cup [−b,−a) : a < b\text{ are real numbers}\}$. Show that $\sigma(S)$ is smaller than the Borel $\sigma$-algebra of $\mathbb R$.

Let $S = \{(a,b] \cup [−b,−a) : a < b\text{ are real numbers}\}$. Show that $\sigma(S)$ is smaller than the Borel $\sigma$-algebra of $\mathbb R$. How can I find a Borel set which is not in ...
1
vote
0answers
21 views

Generic rank of tensors

Let the tensor product of the type $$ \underset{k=1} { \overset{m} \bigotimes } v_k$$ denote a simple tensor. As underlying fields, take $$ \underset{k=1} { \overset{m} \bigotimes } ...