Questions tagged [fa.functional-analysis]
Banach spaces, function spaces, real functions, integral transforms, theory of distributions, measure theory.
8,108
questions
2
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59
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Complex Hölder space
I already posted this question on math.stackexchange, but got no response and was suggested to post it here.
I came across a space in an ergodic theory paper, which I am calling here a (complex) ...
- 121
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0
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62
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Cousin of Fourier transform for rescaling and translating functions
Is there a cousin of the Fourier transform which obeys the following property: if I rescale and translate a function, then its transformed form, at each frequency, only depends on the original ...
1
vote
1
answer
132
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Is Schwartz space $\mathbb R^n$ contained in every fractional Sobolev space on $\mathbb R^n$?
Can someone kindly confirm that the Schwartz space $\mathcal S(\mathbb R^n)$ made of all infinitely-differentiable functions $f:\mathbb R^n \to \mathbb R$ with rapidly decreasing derivatives of all ...
- 5,580
-1
votes
1
answer
40
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Applications and motivations of resolvent for elliptic operator
Let $ A(x)=(a_{ij}(x)):\mathbb{R}^d\to\mathbb{R}^{d\times d} $ be a matrix satisfying ellipticity condition, that is
\begin{align}
\mu^{-1}|\xi|^2\geq \sum_{i,j=1}^da_{ij}(x)\xi_i\xi_j\geq\mu|\xi|^2
\...
0
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0
answers
39
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Norm inequalities for sums of two basic elementary operators
I'm working on this paper. What I'm interested in is this theorem:
where $M_{A,B}(X)=AXB$
I don't know why we can find those two sequences $X_n$ and $x_n$, either I'm finding difficulties to show the ...
1
vote
0
answers
70
views
What's the problem with the evaluation map not being continuous?
When introducing differentiable functions between locally convex spaces, many authors (e.g. Bastiani, Keller, Kriegl-Michor) notice that the evaluation map
$$ E\times E^*\to\mathbb R,\qquad (x,L)\...
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0
answers
135
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Explanation of a step in a preprinted work
I have been studying this preprinted paper and the references therein. I believe that there are some typos; However, since I am not an expert yet, I would like to make sure I am correct.
I do not ...
- 223
3
votes
0
answers
95
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Generalized Jensen's inequality for positively homogeneous functions
The function $f:V \to \hat{\mathbb{R}}$ is said to be positively homogeneous iff $f(\alpha v) = \alpha f(v)$ for every $\alpha \in \mathbb{R}_{++}$. Here $V$ is a real vector space and $\hat{\mathbb{R}...
- 305
3
votes
1
answer
108
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Urysohn's lemma for Bochner functions?
Let $G$ be a locally compact Hausdorff group. In the proof of theorem 4.32 of Folland's book "A course in abstract harmonic analysis", the following result is used:
If $U$ is an open ...
- 261
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0
answers
36
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Some simple conditions on a function $f$ so that $x\mapsto xf(x)$ is convex?
Studying the Braess Paradox for a project at school (with the assiociated Wardrop's equilibria and Nash's game theory result) I came upon one simple question I can not figure out….
If $f$ is a ...
- 1
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votes
1
answer
26
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Closure of the point spectrum of an unbounded diagonalizable operator
Given a (separable) Hilbert space H and an unbounded densely defined linear operator $T:{\cal D}(T) \to $H such that ${\cal D}$ is diagonalizable (it means $\exists$ an O.N.B. of H such that all basis ...
- 677
2
votes
0
answers
33
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duality argument
Throughout my studying for some papers, in particular, the proof of localized Strichart estimates, I encountered the use of the duality argument I could not fully understand. The outline of the ...
- 388
1
vote
0
answers
42
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What is lost after RKHS embedding of the L1 space?
We know that every distribution or $L^1$ function $f$ over space $\mathcal{X}$ (e.g., $R^d$) can be embedded to an RKHS $\mathcal{H}$ with a $1$-bounded kernel $\mathcal{K}$ (e.g., the RBF kernel) ...
- 43
2
votes
1
answer
91
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Convex series and closed convex hulls in normed spaces
Let $(X, \lVert \cdot \rVert)$ be a normed space over $\mathbb{R}$ and $A = \{ a_1,a_2 \ldots \} \subseteq X$ be a closed bounded set.
Let $\overline{\mathrm{co}}(A)$ denote the closed convex hull of ...
- 431
3
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0
answers
101
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Examples of infinite dimensional involutions
Examples of infinite dimensional involutions
I'm looking for more examples of involutions of the type portrayed below, in which two sets of indeterminates (real or complex) each can be transformed ...
- 7,910
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votes
0
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46
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Relation between the norm of Sobolev space $H^1$ and $L^p$ norm for non-increasing radial functions
I am interested to find $$\sup\|u\|_{p}^{p},$$ when $u$ are non-increasing radial functions on the unit ball $B_1$ of $\mathbb{R}^{n}$ such that $$\|u\|_{H1}^2 < r$$ for some $r > 0$.
Since $u$ ...
- 1
4
votes
1
answer
198
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Takesaki lemma 1.16 (volume II, chapter VII)
I am trying to understand the proof of the implication $(i)\implies (ii)$ in Takesaki's book "Theory of operator algebras II", chapter VII, which says the following:
The relevant setting ...
- 261
2
votes
1
answer
71
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Questions about ratio set in a dynamical system
Given a dynamical system $(X, \Omega, \mu)$ ($\Omega$ the $\sigma$-algebra and $\mu$ a measure), we assume there is a group $G$ acting on the system in the sense that, for each group element $g$, $g$ ...
0
votes
1
answer
65
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References and updates on a $L^p$ Factorization theorem by Maurey
In the "Exposé XV" of the 1972-1973 of the Maurey-Schwartz Seminar of functional analysis ("Théorèmes de factorisation pour les opérateurs linéaires à
valeurs dans un espace $L^p(\Omega,...
- 33
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0
answers
77
views
Is the integral functional analytic?
Let $E = C^k(\overline{U})$, $k \geq 1$, be the Banach space of real functions of class $C^k$ on a bounded open set $U \subset \mathbb{R}^n$, with its usual norm. Let $I : E \to \mathbb{R}$ be the ...
- 1,631
3
votes
1
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151
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Explanation of a step in a work by C. E. Kenig and A.D. Ionescu
I am studying the work
Ionescu, A. D.; Kenig, C. E., Local and global wellposedness of periodic KP-I equations, Bourgain, Jean (ed.) et al., Mathematical aspects of nonlinear dispersive equations. ...
- 223
2
votes
0
answers
49
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The Laplace transform and the Lagrange compositional inversion formula
I'm looking for references which derive the Lagrange inversion formula, given below (in bold), for the Taylor series coefficients of the compositional inverse of a function $f$ analytic at the origin ...
- 7,910
0
votes
1
answer
52
views
Strong measurability of operator-valued map induced by a kernel
Let $K\in L^2([0,T]^2)$, and for each $t\in [0,T]$, let $\mathcal{T}_t $ be such that for all $f\in L^2([0,T])$, $\mathcal{T}_t f(s)=\int_0^T K(s,t)K(u,t)f(u)\, d u$ for all $s\in [0,T]$. One can show ...
- 423
3
votes
0
answers
133
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$C^1$-regularity of solution of a Dirichlet problem
I've been stuck trying to hunt down a proof/reference for a certain regularity result which I need for my thesis (all the authors I've consulted refer to it as "well-known"). Consider the ...
4
votes
1
answer
114
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Spectral theory of infinite volume hyperbolic manifolds
I have a question about the discrete spectrum of the Laplace operator on hyperbolic manifolds with infinite volume. I understand the case of infinite area surfaces: see Chapter 7 (Sections 1 and 2) of ...
- 1,071
1
vote
3
answers
200
views
Squeezing more convergence from the convergence in all $L^p$ spaces
Let $X$ be a space endowed with a finite measure $m$. Let $f_n : \to \mathbb C$ be measurable functions such that $|f_n| \le 1$ for all $n$ and $f_n \to 0$ in every space $L^p (X)$ with $p \in [1, \...
- 4,670
5
votes
1
answer
203
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de Rham theorem for tempered distributions
I am wondering if the following statement holds.
If $u\in \mathscr{S}'$ satisfies $\left< u,\Phi\right>=0$ for all $\Phi \in \mathscr{S}$ with $\mathrm{div}\, \Phi=0$, then there exists $p\in \...
- 303
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79
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Self-ajointness of the Laplacian over a Riemannian manifold with boundary
I have some doubts about on a passage found on this article (https://arxiv.org/pdf/1510.08136.pdf).
Let
$(M,g)$ be a Riemannian manifold with boundary;
$E\to M$ be an hermitian fiber bundle;
$\Delta$ ...
2
votes
1
answer
338
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Positiveness of Banach limit [closed]
I‘m currently reading Arveson’s “A Short Course on Spectral Theory”, and I’m stuck at Exercise 3.1 (1). The question is:
Let $l^{\infty}(\mathbb{N})$ be the set of all bounded sequences of complex ...
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99
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+50
Prove comparison principle for $u_t + f(u)_x = g(t,x)$ with $g \in L^\infty(0,T; L^2(\mathbb R))$
Let us consider
$$u_t + f(u)_x = k u_{xx} + g(t,x),$$ with $k>0$, $g \in L^\infty(0,T; L^2(\mathbb R))$ and $f \in W^{1,\infty}(\mathbb R)$ and $f \not \equiv 0$ (possibly, we can also add the ...
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Time evolution in finite-dimensional quantum mechanics [closed]
This question arose more like a curiosity and maybe more suitable for mathstack, but I posted the question there and got no answers, so I am posting it here as well.
Let $M$ be a linear operator on a ...
- 1,025
5
votes
1
answer
125
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Describing the Gamma-transform explicitly in terms of power series
The Gamma transform of a measure is defined as follows. If $\alpha$ is a $\mathbf{Z}_p$-valued measure on $\mathbf{Z}_p$, then the Gamma transform of $\alpha$ is:
$$\Gamma_{\alpha}(s) = \int_{\mathbf{...
- 695
4
votes
2
answers
94
views
Is a unital $*$-morphism from a unital $C^*$-algebra $A$ to $\operatorname{End}_{\mathbb{C}}(K)$ automatically contractive?
Let $A$ be a unital $C^*$-algebra and let $K$ be an inner product space (not necessarily complete!). Let $\pi: A \to \operatorname{End}_{\mathbb{C}}(K)$ be a unital algebra homomorphism such that
$$\...
- 261
1
vote
1
answer
75
views
Hölder continuity of Radon transform of smooth function
Given an integrable function (e.g a probability density function) $f:\mathbb R^n \to \mathbb R$, let $R[f]$ be its Radon transform defined by
$$
R[f](w,b) := \int_{\mathbb R^n} \delta(x^\top w - b)f(x)...
- 5,580
0
votes
0
answers
31
views
Classic Hölder spaces versus Hölder-Zygmund spaces on Riemannian Manifolds
Let $M$ be a compact Riemannian manifold without boundary.
The spaces ${C}^k(M)$ are defined as usual for $k \in \mathbb{N}$ and we can define Hölder spaces ${C}^s(M)$ $s \geq 0$, $s \notin \mathbb{N}$...
- 2,010
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votes
0
answers
34
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Action of fractional Laplacian on Hölder / Besov spaces on Riemannian manifolds
Let $M$ be a compact Riemannian manifold (without boundary) and $\Delta$ be the corresponding (positive) Laplace-Beltrami operator. We also define the operators $I^s = (\mathrm{Id} + \Delta)^{-s}$ for ...
- 2,010
1
vote
0
answers
46
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Sobolev variant of Wasserstein space
Let $\mathcal{P}(\mathbb{R}^n)$ be the set of Borel probability measures on the Euclidean space $\mathbb{R}^n$ and consider thereof consisting of all probability measures $\mathbb{P}$ satisfying $\int\...
- 1
0
votes
0
answers
27
views
Regularity of Volterra convolution integral
I have a question about the regularity of the convolution integral. Let $f: [0,\infty) \to \mathcal{L}(\mathbb{R}^n)$ given by $f(t) = e^{tA}$. Let $g: (0,T) \to \mathbb{R}^n$ such that $g \in L^2(0,T;...
- 1
1
vote
0
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92
views
$C^0$ norm is bounded by $L^{14}$ norm
Let $M$ be a closed manifold of dimension $6$, and we look at the collection of smooth functions on $M$ which satisfy:
\begin{align*}
||f||_{C^0}\leq C\big(||f||_{L^{14}}^2+1\big)
\end{align*}
for ...
- 485
7
votes
1
answer
89
views
Projective tensor product of injective operators
I've seen claims that it is known that for a pair of bounded injective linear operators $T\colon X\to Y, S\colon W\to V$, their tensor product $T\otimes S\colon X \otimes_\pi W\to Y \otimes_\pi V$ ...
- 10.9k
2
votes
0
answers
44
views
Smoothness of Radon transform
Let $f:\mathbb R^n \to \mathbb R$ be density function (i.e nonnegative function which integrates to $1$), and consider its Radon transform $R[f]$ defined by
$$
R[f](w,b) := \int_{\mathbb R^n}\delta(x^\...
- 5,580
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votes
1
answer
83
views
How to calculate the dual spaces of the following spaces?
Let us consider the spaces $C_\infty(E)$, $C_c(E)$, $C_b(E)$, where $E$ is locally compact, $C_\infty(E）$ is all continuous functions vanishing at the ends of $E$, $C_c(E)$ is all the continuous ...
3
votes
1
answer
86
views
Estimate for an oscillatory integral of the first kind
I am confused in finding the right bound for the following oscillatory integral
$$I = \int_\mathbb{R} (\psi(2^{-k} \xi))^2 e^{i (y \xi - 3 \eta \xi^2 t)} d\xi.$$
Where $\psi(2^{-k} \xi)$ is a smooth ...
- 223
1
vote
0
answers
59
views
Properties of Sobolev spaces $W^{k,p}(\Omega, E)$ where $E$ is a Banach space
$\newcommand{\R}{\mathbb R}$Let $E$ be a Banach space with norm $\|\cdot\|_E$ and let $\Omega\subset \R^n$ be an open set.
For $k\geq 0, p\geq 1$ we define $W^{k,p}(\Omega, E)$, the Sobolev space of ...
2
votes
0
answers
41
views
$f\in L^2_k(\mathbb R\times \mathbb S^1)$ implies that $t\mapsto f(t) \in L^2_a(\mathbb R, L^2_{k-a}(\mathbb S^1))$?
$\newcommand{\SS}{\mathbb{S}^1}$
$\newcommand{\R}{\mathbb{R}}$
Consider a function $f:\R\times \SS\to \R$ and suppose that $f$ is in the Sobolev space $L^2_k(\R\times\SS)$ for $k>1$ so that we can ...
3
votes
1
answer
96
views
Impact of annihilators in C*-algebras
Let $A$ be a unital C*-algebra. Let $S\subseteq A$. We put $$\operatorname{Ann}_r(S)=\{a\in A : \forall s\in S,~ ~as=0\}$$
Suppose that $A$ satisfies the following property:
For every $S\subseteq ...
- 3,343
5
votes
0
answers
76
views
Is the Grassmannian of a Banach space an infinite dimensional manifold?
Grassmannian of complemented subspaces in a Banach space is a Banach manifold. This is explained for example in the thesis of Douady and is rather analogous to the finite-dimensional case.
I would ...
- 484
0
votes
0
answers
60
views
Converse to Cameron-Martin theorem
It is known by Cameron-Theorem that if $\mu$ is a centered Gaussian measure on Banach space $\mathcal B$, the equivalent mean-shift measures are exactly the mean-shift by the Cameron-Martin directions....
- 33
4
votes
1
answer
165
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ODE in Banach space
Have I understood this correctly:
So originally we consider the following partial differential equation:
$$u_t= \frac{u_{xx}}{1+w}-\frac{1}{\epsilon}(1+w)u^3+\frac{wu}{\epsilon(1+w)} \text{ in } \...
- 41
3
votes
1
answer
125
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Does property (V) imply the Grothendieck property for dual Banach spaces?
A Banach space $X$ has property (V) whenever for each Banach space $Y$, every unconditionally converging operator $T:X\to Y$ is weakly compact; equivalently, every non-weakly compact operator $T:X\to ...
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