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Banach spaces, function spaces, real functions, integral transforms, theory of distributions, measure theory.

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0answers
12 views

Unclear inequality of L2 norms (Poisson equation for modeling flow)

I encountered a problem working through a paper about modeling flow with the use of the Poisson equation (source given below). There appears an inequality of L2 norms I don't understand so far. Your ...
7
votes
0answers
92 views

Can a positive polynomial on sphere be represented as the sum of squares of spherical harmonics

Let $p\in {\mathbb{R}}[x_1,\ldots, x_d]$ be a homogenous polynomial degree $2n$. We know that if $p$ is positive on $[-\pi,\pi]^d$, $p$ is sum of squares polynomial, i.e. $p$ can be witten as sum of ...
2
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0answers
44 views

Convex hull of piece-wise linear functions

Let $K>1$ be a positive integer. Consider a function class $$\mathcal{F}_K:=\Big\{\max_{1\leq k\leq K} a_k^\top x + b_k:\ ||a_k||\leq 1, |b_k|\leq 1, \forall 1\leq k\leq K \Big\}$$ on some compact ...
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0answers
40 views

Regarding convolution of fejer kernel with a lipschitz function

Let $\mathbb{T}$ be the unit circle in the comlex plain. The Lipschitz class of function on $\mathbb{T}$ is defined here. And the fejer kernel is defined here. If $f\in Lip_{\alpha}(\mathbb{T}), 0<\...
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1answer
49 views

Regarding $\ell_p$ direct sums

I am reading this paper by S.H Karin titled Norm attaining operators and pseudospectrum. In page 2 he gives the definition of $l_p$ direct sum of a family of Banach spaces as follows: If $1\leq p< \...
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0answers
26 views

Moser inequality involving the trace operator

Can anyone give me a reference of whether there exists a Moser-Trudinger type inequality involving the trace operator. More precisely, $$ \int_{a}^{b} \exp\bigg[ \beta U(x, 0)^2\bigg]\leq C$$ for ...
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0answers
37 views

Show that the norm's bound is an exponent

Let $f$ be a continuous function on $S^2$. Consider $g\in C^{\infty}(R)$, such that $g(x)=1$ for $|x|\leq 1$ and for $|x|\geq 2$. Let $h(x)=g(x)-g(2x)$. The notation $proj_k$ denotes the orthogonal ...
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1answer
96 views

Operator power of another operator

I was reading a paper and encountered the following notation: Let $\mathcal{H}=\ell^2(\mathbb{Z})$ and $\{e_p\}_{p\in \mathbb{Z}}$ be an orthonormal basis of $\mathcal{H}$.Define $$ue_p=e_{p+1}\quad ...
8
votes
2answers
249 views

Hölder continuity for operators

Let $x,y$ be positive real numbers then $$|\sqrt{x}-\sqrt{y}|=\dfrac{|x-y|}{\sqrt{x}+\sqrt{y}}=\sqrt{|x-y|}\cdot \dfrac{\sqrt{|x-y|}}{\sqrt{x}+\sqrt{y}}\leq 1\cdot |x-y|^{\frac{1}{2}}$$ we obtain $1/...
13
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2answers
302 views

Structures of the space of neural networks

A neural network can be considered as a function $$\mathbf{R}^m\to\mathbf{R}^n\quad \text{by}\quad x\mapsto w_N\sigma(h_{N-1}+w_{N-1}\sigma(\dotso h_2+w_2\sigma(h_1+w_1 x)\dotso)),$$ where the $w_i$ ...
1
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0answers
111 views

A problem on integrability of derivatives

Let $$f : (0,1) \to \mathbb{R}$$ and $$g(x) = |f(x)|^{r-1} f(x)$$$r \in \mathbb{N}$. It is known that $g\in \mathcal{L}^2(0,1)$ and the $r^{th}$ weak derivative, $ g^{(r)} \in \mathcal{L}^2(0,1)$. I ...
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0answers
233 views

Is that correct $\mathbb R^2\cong\mathbb R$ as measurable spaces? [on hold]

Is that correct $R^2\cong R$ as measurable spaces? If we consider $R$ and $R^2$ with Borel $\sigma$-algebras, is there measurable map from $R$ to $R^2$ with measurable inverse?
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1answer
47 views

Quasinilpotent operator in finite von Neumann algebra

If the trace of all positive powers of a $n \times n$ complex matrix is $0$, then the matrix must be nilpotent. https://math.stackexchange.com/questions/159167/traces-of-all-positive-powers-of-a-...
2
votes
1answer
185 views

Schwartz space on $\bigcup_{n=1}^CR^n$

I have an application where I need to work with the following idea. Let the space $\bigcup_{n=1}^C \mathbb{R}^n$ be associated with the metric $d$ such that for $x=(x_1,\cdots,x_n)$ and $y=(y_1,\cdots,...
2
votes
1answer
61 views

Improving equi-integrability for a family $\mathcal F$ in $L^1(\Omega)$

Let $\mathcal F$ be a weakly compact subset of $L^1(\Omega)$. Dunford–Pettis theorem says that $\mathcal F$ is uniformly integrable. Also, by de la Vallée-Poussin theorem we can find an increasing ...
2
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0answers
50 views

A conjecture characterizing almost uniform convergence of finitely additive conditional probabilities

This question is a continuation of a question I asked a couple weeks ago. Let $(\Omega, \mathcal{C})$ be the Cantor space of binary sequences equipped with the usual product topology, and let $(\...
1
vote
1answer
69 views

Solve nonlinear equation

Suppose that $f:E\to F$(between Banach spaces), is of the form $$f(x)=f(0)+D(x)+N(x).$$ Here $D$ is a linear term, whose kernel is of finite dimension, and admits a right inverse $G$, i.e. $D(G)(\...
3
votes
0answers
50 views

Constant in trace theorem for balls

Consider the standard open ball $B_r:=\left\{x ; \left\lvert x \right\rvert \le R \right\}.$ The trace theorem tells us any function in $W^{k,p}(B_r)$ can be restricted to a function $W^{k-1,p}(\...
8
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1answer
91 views

A relative Kuiper theorem

Let $(H_0, \langle \,,\,\rangle_0)$ be a real separable Hilbert space, and let $(H_1, \langle \,,\,\rangle_1)$ be a Hilbert space such that $H_1 \subset H_0$ is dense and such that the inclusion $(...
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0answers
54 views

Nonlinear maps in Riesz Thorin theorem

The Riesz Thorin theorem allows us to interpolate between $L^p$ spaces and the usual assumption is that the map $T$ is linear. What I was wondering about is whether this is because otherwise you do ...
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0answers
47 views

Proof of Lemma 7.1 Bonsall and Duncan

In the proof of Lemma 1 in section 7 (A functional calculus for single Banach algebra element) of the book Complete normed algebras by Bonsall and Duncan, the last line says $$\phi\left(\frac{1}{2\pi ...
2
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1answer
120 views

Function is $L^p$-integrable for $p >1$ [Kähler Geometry]

I am reading through a proof in W. Ding and G. Tian's 1992 paper on the generalised Futaki invariant. To provide context, we are looking for obstructions to the existence of Kähler--Einstein metrics ...
3
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0answers
98 views

Diffeomorphism group action on the space of embeddings

Let $S$ and $M$ be two finite-dimensional smooth manifolds with $\dim S\le \dim M$. Then it is known (e.g.Kriegl-Michor's book) that the set $\mathrm{Emb}(S, M)$ of all smooth embeddings $S\to M$ is ...
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0answers
54 views

Is a relatively weakly compact subset of $W^{1,1}(\Omega)$ metrizable?

Let $\Omega$ be a domain with smooth boundary. Let $S\subset W^{1,1}(\Omega)$ be a relatively weakly compact set. Is it true that $(S,w)$ is metrizable? Since $S$ is relatively weakly compact, it ...
1
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1answer
60 views

Steklov averages and negative parabolic sobolev spaces

Suppose one is given a function $$ w \in L^p(0,T;W^{1,p}(\Omega)) \qquad \text{and} \qquad \frac{dw}{dt} \in L^{p'}(0,T; W^{-1,p'}(\Omega)) $$ I am interested if the following holds: Denote the ...
11
votes
1answer
365 views

Nonlinear Schrödinger equation with discrete Laplacian

In the paper "Global existence and scattering for rough solutions of a nonlinear Schrödinger equation on $\mathbb{R}^3$" by Colliander, Keel, Staffilani, Takaoka and Tao it is argued in the beginning ...
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0answers
44 views

Sobolev embeddings for vector-valued functions

I would like to know if there is a simple extension of the standard Sobolev embeddings for functions taking values in another Euclidean space. In particular, let $\Omega \subset \mathbb{R}^n$ be a ...
1
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1answer
81 views

The tensor product of two bounded operators

Let $E$, $F$ be two complex Hilbert spaces and $\mathcal{L}(E)$ (resp. $\mathcal{L}(F)$) be the algebra of all bounded linear operators on $E$ (resp. $F$). The algebraic tensor product of $E$ and $F$ ...
5
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2answers
364 views

Existence of Solution, System of Equations

Suppose $P(\lambda, i)$ is the probability that a Poisson random variable with average $\lambda$ is equal to $i$, i.e. $\frac{\lambda^i}{e^{\lambda}i!}$ I think the following system of equations ...
0
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1answer
56 views

Harnack inequality for fractional laplacian

Let u be a positive solution of $s\in (0, 1) $ \begin{equation} \left\{\begin{aligned} (-\Delta )^{s} u &= 0 \text{ in } (-2T, 2T)\\ u &=g\quad\text{in}\quad \mathbb R\setminus(-2T, 2T). \...
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votes
1answer
66 views

Compatibility of the absolute value with the integration

Let $H$ be a separable Hilbert space and $L^{1}(H)$ be the space of trace class operators on $H$. Let $f:\mathbb{R}\to L^{1}(H)$ be a measurable function that is $t\to \operatorname{tr}(f(t))$ is ...
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0answers
90 views

Support of a microlocal defect measure

I'm trying to complete the proof of the Theorem 6.1 in the notes https://www.math.u-psud.fr/~nb/articles/coursX.pdf, which ensures, under certain conditions, that the support of the microlocal defect ...
3
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0answers
41 views

PDE satisfied by projection of a function onto a subspace

Given an open bounded set $D\subset \mathbb R^N$, let $f\in W^{-1,q}(D)$ and let $u$ be a Sobolev function $u\in W_0^{1,p}(D)$ such that $u$ solves the PDE $$ \begin{cases} -\Delta_p u=f\;\text{in $D$}...
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55 views

Are invertible measures strictly dense?

Let $L_1(\mathbb T)$ be considered as a closed ideal of $M(\mathbb T)$, the Banach algebra of measures on the circle. Then $M(\mathbb T)$ can be identified with the multiplier algebra of $L_1(\mathbb ...
7
votes
1answer
452 views

If $E\oplus_\phi E \cong E\oplus_\psi E,$ does it imply that $\phi= \psi$?

Let $E\neq \{0\}$ be a Banach space. For each $p\in[1,\infty), $ we define $$E\oplus_p E = \{(x,y): x\in E, y\in E, \|(x,y)\| = \sqrt[p]{\|x\|^p + \|y\|^p}\}.$$ Let $F$ be another Banach space. By $E\...
5
votes
2answers
148 views

VC dimension, fat-shattering dimension, and other complexity measures, of a class BV functions

I wish to show that a function which is "essentially constant" (defined shortly) can't be a good classifier (machine learning). For this i need to estimate the "complexity" of such a class of ...
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0answers
44 views

Set of functions orthogonal to $ (a - b x)^{c_n} $ [on hold]

What is $v_n(x)$, s.t. $\int_{-1}^{+1} v_n(x) u_n(x) dx = \delta_{nm}$ or $\int_{-1}^{+1} v(k', x) u(k, x) dx = \delta(k-k')$, with $u_n(x) = (a-b x)^{c_n}$, $c_n$ discrete in the first, ...
5
votes
2answers
207 views

Vector valued disc “algebra”

I am interested in a vector-valued form of the disc "algebra" (which in this setting is not in general an algebra, hence the scare quotes). Let $E$ be a Banach space, and let $A(\mathbb D,E)$ be the ...
0
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1answer
99 views

Regarding exponential in a Banach algebra

Let $A$ be a complex unital Banach algebra. Let exp$(A)$ denote the range of the exponential function on $A$. Now exp$(A)$ lies in the set of all invertible elements of $A$ (denoted by $G(A)$). Can ...
7
votes
2answers
194 views

Non-separable metric probability space

Let us say a metric probability space $(X,\rho,\mu)$ has property (*) if: the support of $\mu$ is contained in a separable subspace of $X$. Questions: 1. Is there a standard name for this property? ...
4
votes
1answer
266 views

What is the consistency strength of non-existence of outer automorphisms of Calkin algebra?

The Calkin algebra $C(H)$ is the quotient of $B(H)$, the ring of bounded linear operators on a separable infinite-dimensional Hilbert space $H$, by the ideal $K(H)$ of compact operators. In 1977, ...
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0answers
69 views

For what functions does Nash inequality becomes equality?

For what functions does Nash inequality becomes an equality? Also any comment on the regularity of these functions (weak solutions to equality)? Also same question about Poincare inequality.
4
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0answers
68 views

Representation on square integrable sections of a principal bundle

Let $X\rightarrow Y$ be a smooth principal $G$-bundle for some Lie group $G$. Then $L^2(X)$ has a natural $G$-action determined by fibrewise action of $G$ on $X$. We have an abstract isomorphism of ...
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0answers
58 views

Iterative methods for minimizing sequences

Let $\mathbb{X}$ be a Banach space equipped with some norm $||\cdot||_\mathbb{X}$ and $F:\mathbb{X}\to\mathbb{R}$ be some linear functional. Suppose we are given a set $A\subseteq\mathbb{X}$ which is ...
3
votes
2answers
168 views

Decay estimate for the heat equation: $\sup_{t>0}\int_{\mathbb{R}} t^\alpha |u_x|^2\ dx$

Let $u$ be a solution of the heat equation $$u_t - u_{xx} = 0, \quad t>0, x \in \mathbb{R}$$ with initial data $u(0,\cdot) = u_0$. Fix $\alpha >0$. How can I estimate (without using explicitly ...
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0answers
37 views

Dual space of polynomial one-form

Recently I read a paper "Quasi-particles models for the representations of Lie algebras and geometry of flag manifold". In section 2, author gives a fact without proof. Now I rephrase this fact as ...
4
votes
0answers
149 views

Is the Gelfand transform strictly continuous?

Let $M$ be the Banach algebra of measures on the circle with $L_1$ naturally sitting as a closed ideal of $M$. Then $M$ carries the strict topology implemented by the family of seminorms $\|\mu\|_f = \...
2
votes
0answers
170 views

Absence of fixed points

Let $f$ be an arbitrary function in $L^2(0,\infty)$ and consider the function $$(g_f)(y) = \frac{1}{y-x_0} \int_{0}^{\infty} f(x) \frac{xy}{(x^2+y^2+1)} \ dx$$ where $x_0$ is an arbitrary but fixed ...
19
votes
1answer
1k views

How do mathematicians and physicists think of SL(2,R) acting on Gaussian functions?

Let $\mathcal{N}(\mu,\sigma^2)$ denote the Gaussian distribution on $\mathbb{R}$: $$ \mathcal{N}(\mu,\sigma^2)(x) = \frac{1}{\sqrt{2\pi\sigma^2}} e^{-\frac{(x-\mu)^2}{2\sigma^2}}.$$ A Gaussian ...
2
votes
0answers
48 views

Are sums extremal for subgaussian concentration?

Bobkov and Houdre https://projecteuclid.org/euclid.bj/1178291721 showed that among all $f:R^n\to R$ that are $1$-Lipschitz with respect to the $\ell_1$ metric, the variance is maximized by sums. ...