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Inverse Fourier transform of $\frac{1}{\sqrt{\xi_1} + \xi_2}$

Consider the inverse Fourier transform of $\frac{1}{\sqrt{\xi_1} + \xi_2}$. My question is, how can we conclude about the decay properties, support and smoothness of the inverse Fourier transform? I ...
anonymous's user avatar
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152 views

The Lebesgue measure of the low level sets of the two-dimmension Fourier transform of a compactly supported function

Let $f\in {{L}^{1}}\left( {{\mathbb{R}}^{2}} \right)$ be a density function with the support $\operatorname{supp}\left( f \right)\subset \left[ a,b \right]\times \left[ c,d \right]$. Denoted by $\hat{...
Baily's user avatar
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121 views

A special approximation of BV functions

Suppose $\Omega:=B(0,1)\subset \mathbb R^2$. Let $u\in BV(\Omega)$ be a radially symmetric, i.e., $u(x)=u(Rx)$ for all $R\in SO(2)$. In addition, suppose $w$ to be an affine function,. i.e., $\...
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Must the Lebesgue measure of a $\rho$ - neighbourhood of an $(n-2)$ - dimensional set be at least $c\rho^2$?

The Lebesgue measure of a $\rho$-neighbourhood of a point in $\mathbb{R}^2$ is of course equal to $c\rho^2$. Similar such considerations in higher dimensions lead me to the following question: Given ...
Spencer's user avatar
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145 views

Discrete measures and discrete kernels

This is a cross-post from math.stack. Let $d\in\mathbb N$ and $\mu$ be the probability measure on $\mathbb R^d$ defined by $\mu=\sum_{k=1}^\infty 2^{-k}\delta_{x_k}$ for some sequence $(x_k)_{k\in\...
andy teich's user avatar
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68 views

Estimate bounds on Minkowski distance from point to a segment in Lp space

Assumptions Let $L_p(x,y)=(\sum_i|x_i - y_i|^p)^{1/p}$ (Minkowski metric), $a,b$ be arbitrary $n$-dimensional points, $c$ be a point that satisfies $L_p(a,b) = L_p(a,c) + L_p(c,b)$, i.e., a point ...
tomash's user avatar
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Do they have the same limit?

Suppose $a(\cdot)\in L^p$ and is symmetric and $b(\cdot)\in L^q$, where $1/p+2/q=2$, $p,q\ge 1$. Consider the quantity $Q_T=$ $$ \frac{1}{T}\int_{\mathbb{R}}dx\int_{[-T,T]^2}d\mathbf{v}\int_{[-T,T]^2}...
Uchiha's user avatar
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161 views

Asymptotic analysis of a sum of complex summands using integral

I'm trying to find the exact asymptotics of a sum: $$A = \sum^n_{i=0} \begin{pmatrix} 2n \\ i \end{pmatrix} x^{i} y^{2n-i} $$ as $n\rightarrow\infty$. Here $x,y$ are complex numbers, $|x|\leq1, |y|\...
teagut's user avatar
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A question about the duality principle

Suppose $X$ and $Y$ are finite sets and $K:X\times Y\to \mathbb R$ is some function. We get an integral transform from the space of real functions on $X$ to real functions on $Y$ given by $$\Phi_Kf(y)=...
brando's user avatar
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182 views

Establishing an upper bound for a dyadic average of a function in $ {L^{p}}([0,1)) $

Suppose that $ f $ is $ 1 $-periodic and that $ f \in {L^{p}}([0,1)) $, where $ p > 1 $. Let $$ (D_{n})_{n \in \mathbb{N}_{0}} = \left( \left\{ I^{n}_{j} \stackrel{\text{df}}{=} \left[ \frac{j}{2^{...
sokho's user avatar
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153 views

extension of function in an abstract metric space

my question is the following.(Maybe my title is not quite proper for this question): Let $(E,d)$ be a Polish space (or a separable metric space), let $\xi: E\to R_+$ be a Lipschitz function. Now set $...
CodeGolf's user avatar
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Does there exist this special kind of homeomorphism?

Let $A,B\subset\mathbb{R}^n, n\geq 2,$ are two different shaped spindles. One is thick and one is thin. (Sorry for my unprofessional statements. Unsure about how to say it rigorously.) So there are ...
Lucy's user avatar
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206 views

About approximate eigenvalue

I am in trouble when read the book "D.Henry, Geometric Theory of Semiliner Parabolic Equations". The question is relate to Page 104,proof Lemma 5.1.4. Suppose $X$ is a real Banach Space, $M$ is a ...
user44565's user avatar
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428 views

Given an even function how to obtain the most close odd function and vise versa?

Given an even function $f(x)$, how to obtain the most close to it continuous odd function $g(x)$? By most close I mean that $\int_0^\infty |f(x)-g(x)| dx$ be the minimum possible and the difference $|...
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45 views

compactness related to some distance defined on the space of increasing functions2

Let $I=[0,1]$ and denote by $C^{+}(I)$ the space of continuous increasing functions. Can we find a distance $d$ for $C^+(I)$ such that the set of the form $$d(f,g)\rightarrow 0\Longrightarrow f(1)\...
CodeGolf's user avatar
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405 views

Dual of the space of vector valued Borel measures

What is the dual of the space of all vector valued Borel measures?
Weymon He's user avatar
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115 views

Quasi-simmetric function and bi-Lipschitz functions

Assume that $f$ is a homeomorphism of the unit circle onto itself. If $$1/M \le \frac{|f(e^{i(t+s)})-f(e^{i(t)})|}{|f(e^{i(t)})-f(e^{i(t-s)})|}\le M,$$ then we say that $f$ is $M-$quasi-symmetric ...
user36162's user avatar
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100 views

Two distribution spaces ${\mathcal S}'/{\mathcal P}$ and ${\mathcal S}_\infty'$

Let ${\mathcal S}'$ be the set of all distributions. Denote by ${\mathcal P}$ the set of all polynomials, which is embedded into ${\mathcal S}'$ as a closed subspace. Equip ${\mathcal S'}/{\mathcal P}$...
Yoshihiro Sawano's user avatar
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60 views

Relative homology of interlevel set

Let us consider a function $f:\mathbb{R}^3→\mathbb{R}$, $f(x,y,z)=x^3+y^3+z^3-5yz$. Can anybody drop a hint how to compute relative homology of interlevel sets with coefficients in $\mathbb{R}: H_{\...
quantum's user avatar
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127 views

A question of the weights $A_\infty$' equvalent condition in Real &Harmonic analysis

I have a question. The question is to prove: The weight $w \in A_\infty $if and only if $\frac{1}{|Q|}\int_Q w(x)dx \cdot \exp\left(\frac{1}{|Q|}\int_Q \log\frac{1}{w(x)}dx\right)\leq C$, for all ...
Reigion Ho's user avatar
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94 views

Extending coverings over dense subsets

Let $X$ be a metric space with $D⊆X$ a dense subset. If there is a covering for $D$, under which conditions on the covering is it possible to guarantee that the covering also covers $X$? For a ...
Michael's user avatar
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149 views

Does this sequence of H\"older functions have a limit?

Let $\left\{\alpha_{n}\right\}_{n\in \mathbb{N}}$ a sequence of positive real numbers with $$\alpha_{n}\in (0,1)\quad \textrm{and}\quad \alpha_{n}>\alpha_{n+1}$$ Moreover suppose $$\lim_{n\...
student's user avatar
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490 views

Sufficient conditions for continuity of function $y\mapsto\min_{[x_0,y]}\phi$

Let $\phi:\mathbb{R}\to\mathbb{R}$ a continuous function. Fix $x_0\in\mathbb{R}$ and consider $$\psi:\mathbb{R}\to\mathbb{R},\ \psi(y)=\min_{\xi\in[x_0,y]}\phi(\xi)\ .$$ Is $\psi$ a continuous ...
user22980's user avatar
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241 views

Continuity of a function

Let $f\in L^2(\mathbb{R}^3)$ with compact suppport and $z\in\mathbb{C}$. Is the following function continuous for $z\in Q = \{ z : \Re z\in [a,b], \Im \sqrt{z} \in (0,1] \}$: $$ F(z)=\bigg(\alpha-i\...
Mario's user avatar
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92 views

Class of integrable 0/1-functions "with no null sets."

I am looking for (the name of) a class of functions from $\mathbf{R}^2$ $(\mathbf{R}^n)$ to {0, 1} that are integrable. Let $f$ be in this class and $E$ be the set of all points where $f$ is equal to ...
Ben's user avatar
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244 views

Checking whether this would be bounded

It may be better to post this question here. Assume that $M$ is an $m$ by $m$ ($m$ is an even number) symmetric positive-semi-definite matrix with exactly $m/2$ positive eigenvalues and every entry of ...
io0's user avatar
  • 1
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382 views

Lambert W-function

I asked this question MSE, but didn't get any answers. Maybe here someone can help. I need to solve $$ \theta \rho^{\theta}+r \theta>v $$ where $\theta \in \mathbb{R}^{+}, -1 < r,v<1, \ 0&...
Alex's user avatar
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0 answers
183 views

Continuity of the Shadow of a Nondecreasing Function

So I'm working a lot with monotone nondecreasing functions $f : [0,1] \rightarrow [0,1]$, and I'm defining a certain discrete dynamics on them. Here nondecreasing means $x < y \Rightarrow f(x) \leq ...
A Blumenthal's user avatar
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0 answers
176 views

search for a function satisfying some conditions

Hi everyone, I would like to find a function $$\Psi\in\mathcal{C}^2: z\in\mathbb{R}\rightarrow\Psi(z)\in\mathbb{R_+}$$ satisfying the following conditions: $$1-\frac{z\Psi'(z)}{\Psi(z)}+8s\Psi''(z)...
Higgs88's user avatar
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193 views

Boundedness of Riemann-like sums on unbounded interval

Hi I am trying to find suitable conditions (integrability, growth...) on a function $f:\mathbb{R}\to \mathbb{R}$ such that: \begin{equation} \sum_{k\in\mathbb{Z}}f(kh)h= \mathcal{O}(1),\qquad h\to 0^+...
Francesco Mina's user avatar
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0 answers
92 views

Lower bound for double sums with power law decay terms.

This question is related to a work in progress about Ballistic-Diffusive phase transition for some random polymers with long range self-repulsion. The motivation to ask here if the inequality below ...
Leandro's user avatar
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109 views

Expansion (asymptotic) of scalar function of a square matrix , in terms of determinant of argument?

The title says it all. I have a scalar function (really, a determinant) of a square matrix argument. Can I find an (asymptotic) expansion of the function, in a series in the determinant of the ...
kjetil b halvorsen's user avatar
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0 answers
345 views

Jacobian of the inversion map

Let $F:Tr(n,\mathbb{R})\cap GL_n(\mathbb{R})\rightarrow Tr(n,\mathbb{R})\cap GL_n(\mathbb{R})$ be the map which sends a matrix $A$ to its inverse $A^{-1}$. If we consider $F$ as a function from $(\...
Diego Sulca's user avatar
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0 answers
700 views

Sigma algebra generated

Let $\mathcal{L} \subset \mathbb{R}$ the Lebesgue sigma algebra and $\mathcal{B} \subset \mathbb{R}^{n}$ the Borel sigma algebra. I'll denotes by $\mathcal{L} \times \mathcal{B}$ the smallest sigma ...
Santos's user avatar
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0 answers
165 views

minimizing the integral of a function over square sets.

Hi! I'm interested in some problems, but to be honest i'm not sure of the field they belong to. Let $h(x,y)$ be a bivariate function on $X^2$, where $X$ is some nice topological space (for instance $...
kaleidoscop's user avatar
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1 answer
122 views

Existence of an eigenpair for d-bar operator in the unit disck

Let $\overline{\partial}=\frac{1}{2}(\partial_{x}+\textrm{i} \,\partial_y)$ and let $D$ be the unit disc in the complex plane. For each $\lambda \in \mathbb C$, consider the problem: $$ \overline{\...
Ali's user avatar
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-1 votes
0 answers
132 views

Trig conjecture about square roots and Arcsin

Let $r(a,b)$ be a rational number depending on positive integers $a,b$ and $r(a,b)$ being nonnegative. For every $b$ there is an $a$ such that $r(a,b)$ is not $0$. Let $C(b)$ be a squarefree positive ...
mick's user avatar
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-1 votes
0 answers
51 views

Existence of continuous crossection

Let $G$ be a second countable locally compact Hausdorff groupoid. Then $G= \sqcup_{u\in G^{0}}G^{u}$ where $G^{0}$ is the unit space of $G$ and $G^{u}=r^{-1}(u)$. Here $r:G \to G^{0}, r(x)=xx^{-1}$. ...
K N SRIDHARAN NAMBOODIRI's user avatar
-1 votes
2 answers
87 views

Limits of integral series

Suppose we have the series of functions: \begin{equation} F(x)=\sum_{n=1}^{\infty} f_n(x) \end{equation} where convergence is uniform. Additionally, consider the partial functions of the series: \...
george andrade's user avatar
-1 votes
1 answer
550 views

Lower bound of an expectation

Suppose a random variable $X$ has unit variance i.e. $\sigma^{2} = 1$. Is there a positive constant $c > 0$ such that $$\mathbb{E}[\ | X - \mathbb{E}[X] | \ ] \ge c $$ My attempt of a solution is ...
NebulousReveal's user avatar
-1 votes
1 answer
74 views

Example(s) where replacing a multivariate, discrete RV with a single, univariate RV fail

Let $X_1,\ldots,X_n,Y,Z$ be $n+2$ binary random variables and define $X=(X_1,\ldots,X_n)$. In most problems, instead of treating $X$ as $n$ distinct binary random variables, there is no loss of ...
user3312's user avatar
-1 votes
1 answer
236 views

Natural candidates for sub-half-exponential which limit to half-exponential function from below

There are no closed form candidates for half-exponential functions "Closed-form" functions with half-exponential growth. However sub-half-exponentials (functions whose composition grows ...
VS.'s user avatar
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-1 votes
1 answer
193 views

Limit of the convolution of derivative of Gaussian heat kernel

I'm looking for the following limit: $$\lim_{\varepsilon\to 0^+}\int_{-\sqrt{\varepsilon}}^{\sqrt{\varepsilon}}\frac{1}{\sqrt{2\pi}\varepsilon^{3/2}}\left(-1+\frac{x^2}{\varepsilon}\right)e^{-\frac{x^...
yassine yassine's user avatar
-1 votes
1 answer
126 views

Is there a name for this family of integral?

This one: $\int_{0}^{\bar{x}}e^{-x^{a}}x^{b}(1-x)^{c}dx,a,b,c\ge0$. When $a=1,c=0,\bar{x}=\infty$ it is the gamma function.
ZUN LI's user avatar
  • 101
-1 votes
1 answer
180 views

Orthogonal polynomials of the second kind

Let $L: \mathbb{R}[x] \rightarrow \mathbb{R}$ be a positive definite linear functional and let that $\{s_n\}$ be a positive semi-definite sequence such that $L(x^n)= s_n, n\ge 0.$ Given a positive ...
Jaynot's user avatar
  • 1
-1 votes
1 answer
63 views

Idempotent solutions to the implict function theorem other than the identity?

I am interested in the following problem. Assume that an (anti)symmetric function $g:\mathbb{R}^2 \to \mathbb{R}$ satisfies the implicit function theorem. That is, $g(x,y) = \pm g(y,x)$ and $g(x,y)=0$ ...
Charlie's user avatar
-1 votes
1 answer
173 views

For a given $n$, under what condition(s) there exists (at least) two different $c$ and $c′$ such that $X_n^c=X_n^{c'}$

Let $X_n^c=\{\cos\left((4k-c)\frac{\pi}{2n}+\frac{\pi}{4}\right): k=0, 1, \dots, n-1\}$ where $c\in\{0, 1, \ldots, \lfloor\frac{n}{2}\rfloor\}$ and $n$ is any positive integer greater than 3. I want ...
G_0_p_i_e's user avatar
-1 votes
1 answer
148 views

Analytic extension of the exterior Newtonian potential into the domain

I just want to know for a domain with smooth boundary whether exterior Newtonian potential has singular analytic extension into the domain or into a part of the domain. Definition of Newtonian ...
user60554's user avatar
-2 votes
0 answers
64 views

A Problem using Limits of Sequences of Functions

Suppose $\{f_n\}$ is a sequence of nonnegative extended real-valued functions on $X$ and $\lim_{n\to\infty}f_n=f$. Take a simple function $0\leq\varphi\leq f$. If $X_{\infty}=\{x\in X: \varphi(x)=a>...
hunter's user avatar
  • 1
-2 votes
1 answer
208 views

Strong estimates for the zeta function on natural numbers

Let $$\zeta(s) = \sum_{n = 1}^\infty \frac{1}{n^s}$$ be the Riemann zeta function (here we just consider real $s$). We do have a description given by $$\zeta(s) = \frac{s}{s-1}-s\int_{1}^\infty \frac{...
tobias's user avatar
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