# Questions tagged [fa.functional-analysis]

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

5,629 questions

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121 views

### A maximization problem

Consider the minimization problem described this paper. Let $f_{\lambda}$ be the minimizer. As a part of extending my work, I am able to show the following facts
$$\lim_\limits{\lambda \to 0}\|f_{\...

**5**

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**0**answers

83 views

### How to calculate the volume of a parallelepiped in a normed space?

Let $E$ be a real normed space, and let $v_1,...,v_n\in E$ be linearly independent. The parallelepiped defined by these vectors is $P=\{\sum_{i=1}^{n}\alpha_i v_i|~0\le\alpha_i\le 1\}$. Since $E$ is a ...

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42 views

### The tower of path algebras associated to a tower of finite dimensional $C^*$-algebras is isomorphic to the original tower

Let $A_0\subseteq A_1\subseteq...$ be an infinite tower of unital inclusions of finite dimensional $C^*$-algebras and $B_0\subseteq B_1\subseteq ...$ be its associated infinite tower of path algebras. ...

**3**

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**1**answer

132 views

### Simple proof of Prékopa's Theorem: log-concavity is preserved by marginalization

The following result is well-known:
Suppose that $H(x,y)$ is a log-concave distribution for $(x,y) \in \mathbb R^{m \times n}$ so that by definition we have
$$H \left( (1 - \lambda)(x_1,y_1) + \...

**2**

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**0**answers

63 views

### Exp-decay estimate of Schrodinger equation

Consider the equation $Hu=0$ with $u\in L^2(\Omega)$, where $H=-\Delta+V$ for some bounded continuous function $V$ and $\Omega$ is an un-bounded domain(e.g. $\mathbb R^n$). If $0$ is in discrete ...

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**0**answers

79 views

### Functional Analysis book with graphical descriptions [on hold]

I am looking for some book about functional analysis that has also graphical descriptions and images of the important concepts.
Anyone knows some book like this?

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**0**answers

36 views

### Show convergence of a sequence of resolvent operators

Let
$E$ be a locally compact separable metric space
$(\mathcal D(A),A)$ be the generator of a strongly continuous contraction semigroup on $C_0(E)$
$E_n$ be a metric space for $n\in\mathbb N$
$(\...

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**0**answers

22 views

### Is there an easy characterisation (perhaps some generalised Löwner representation) for operator monotone functions of order $n$?

As per my understanding, roughly stated, $f$ is an operator monotone function of order $n$ if for all $n\times n$ (Hermitian) matrices, $X,Y\ge0$ which satisfy $X\ge Y$, we have $f(X)\ge f(Y)$.
If $f$...

**2**

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61 views

### Gagliardo-Nirenberg inequality for periodic functions?

I am interested in Gagliardo-Nirenberg type inequality (see https://en.m.wikipedia.org/wiki/Gagliardo%E2%80%93Nirenberg_interpolation_inequality) for functions in the space
$$H^1_T(\mathbb{R}^n)=\...

**6**

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**4**answers

199 views

### Open mapping theorem for complete non-metrizable spaces?

The classical open mapping theorem in functional analysis certainly holds in the Banach space setting, and this is where I first encountered it. Slightly more advanced textbooks (e.g. Rudin's ...

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**0**answers

32 views

### Time derivative in parabolic Hölder spaces

Let $\Omega$ be a regular open set in $\mathbb{R}^n$ and $T>0$.
Let $C^{\frac{1+\alpha}{2};1+\alpha}([0,T]\times \overline{\Omega})$ be the space of functions $f$ which are $\frac{1+\alpha}{2}$-...

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**0**answers

32 views

### $W^{k,p}$ and Holder regularity for linear elliptic systems with Neumann boundary data

I'm looking for a text or paper that discusses regularity in the Sobolev and Holder sense for general linear elliptic systems of PDEs on bounded domains with Neumann boundary data.
The book by ...

**4**

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**1**answer

126 views

### Relation between two different functionals: $\lVert p^{-\max}_{-\varepsilon}\rVert$ and $\kappa_{p}^{-1}(1-\varepsilon)$

Given a non-negative sequence $p=(p_i)_{i\in\mathbb{N}}\in \ell_1$ such that $\lVert p\rVert_1 = 1$,we define the two following quantities, for every $\varepsilon \in (0,1]$.
Assuming, without loss ...

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**0**answers

86 views

### Korovkin subset of $C(\mathbb{T})$

Let $K$ be a compact Hausdorff space and $A$ be a subset of $C(K)$. $A$ is said to be a Korovkin set if for every sequence $(T_n)$ of positive linear operators on $C(K)$, the condition $\|T_n(f)- f\|_\...

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**1**answer

183 views

+50

### Non-uniform property of sequences

Let us say a sequence $(x_n)_{n=1}^\infty$ in some Banach space $X$ has $S_C$ if there exist $k_1<k_2<\ldots$ such that for any $t\in \mathbb{N}$ and scalars $(a_n)_{n=1}^t$, $$\|\sum_{n=1}^t ...

**4**

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**1**answer

106 views

### Comparison of several topologies for probability measures

Let $X$ be a compact metric space and denote $\mathcal M(X)$ the set of probability measures on $X$. For $\mu\in\mathcal M(X)$ we write $\operatorname{supp} \mu$ for the support of $\mu$. As is well ...

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**0**answers

40 views

### Strong Differentiability of Spectral Projections

Let $H$ be a Hilbert space and $W$ be a dense subspace, equipped with a different norm that turns it into a Hilbert space. Let $(A(t))_{t\in[0,T]}$ be a family of Operators in $B(W,H)$ (bounded ...

**5**

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**1**answer

139 views

### Textbook recommendations: Weakly almost periodic functions

I am currently studying Ergodic Theory from Glasner’s book - in it, weakly almost periodic functions play a large role, as well as general “means” and unitary representations of groups on Hilbert ...

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**0**answers

31 views

### Convergence of regression coefficients to probability density

By simulation we create a vector $Y = (y_1,y_2,...,y_n)$, where each $y_i \in R$ is independently drawn from a given non-degenerate distribution.
Next we create by simulation a vector $\xi = (\xi_1,\...

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**1**answer

47 views

### Box counting dimension of a set and Lipschitz functions

If $f$ is Lipschitz, then the following holds for the Hausdorff dimension:
$$\dim_H f(A) \le \dim_H A.$$
Is the same true for the box counting dimension?

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41 views

### Box counting dimension of the graph of the Cantor function

Consider the Cantor staircase function. The Hausdorff dimension of its graph is $1$. What is the box-counting dimension of its graph?
A more general question is Box counting dimension of the graph of ...

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47 views

### An inequality for norms involving convolutions [migrated]

I need to prove/disprove the following existence theorem:
$$
\forall \epsilon > 0, \quad \exists f\in L^1(\mathbb R), g\in L^p(\mathbb R) : \quad (1-\epsilon)\|f\|_1\|g\|_p < \|f\ast g\|_p,
$$
...

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**0**answers

101 views

### Is this integral zero?

I'd like to know if one integral expression I have can be shown to be zero for all possible cases. Let me introduce some notation.
Consider $\mathfrak{g}=C^{\infty}(M)$ and the dual $\mathfrak{g}^*=\...

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**1**answer

111 views

### Hausdorff dimension of the graph of the sum of two continuous functions

How can one prove the following result on the Hausdorff dimension of the graph of the sum of two continuous functions:
Let $f,g:[0,1] \to \mathbb R$ be two continuous functions. Suppose that $$\...

**3**

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**1**answer

216 views

+100

### Norm of two operators on $l^2(\mathbb{Z}_{2}*\mathbb{Z}_{2})$ different

In my research I encounered the following (very concrete) question: Consider the (discrete) group $G:=\mathbb{Z}_{2}*\mathbb{Z}_{2}$. Let $s\text{, }t\in G$ be the generating elements and define for $\...

**2**

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**0**answers

90 views

### Euler-Lagrange equations on a differentiable manifold

I am following the conventions of https://arxiv.org/abs/math-ph/9902027
Let $M$ be a differentiable manifold, $E \to M$ a vector bundle over $M$ with fibre $F$, $J^1(E)$ the rank-one jet bundle over $...

**3**

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**1**answer

133 views

### Uniform convergence of generalised Fourier series

Suppose $u_n$ is an orthonormal basis of smooth functions on $S^1$.
Does there exist a smooth function $u$ such that the generalised Fourier series
$$u=\sum_{n\in\mathbb{N}} \langle u,u_n\rangle u_n ...

**3**

votes

**1**answer

114 views

### When does the spectral radius strictly increase?

For bounded linear operators $A$ and $B$ on a Banach space $X$, I'm looking for results which imply that $r(A) < r(A+B)$ (note the strict inequality), where $r(A)$ denotes the spectral radius of $A$...

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**0**answers

39 views

### Derivation of the vortex filament equation from Euler equation

How can the vortex filament equation
$$\partial_t \chi = \partial_s \chi \wedge \partial_{ss} \chi,$$
where $\chi(t,s)$ is a curve in $\mathbb R^3$,
be derived from the Euler equation
$$\partial_t \...

**5**

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**0**answers

43 views

### First Cousin Problem for Bergman spaces

I recall (an easy case of) the first Cousin problem :
Let $\Omega_1, \Omega_2$ be two open subsets of the complex plane
$\mathbb{C}$ with non-empty intersection and $f$ be holomorphic on $\...

**3**

votes

**1**answer

44 views

### Reference Request: $L^p(x)$/(Musielak–Orlicz space) analogue of classical $L^p$ result

Fix a non-empty open domain $\Omega\subseteq \mathbb{R}^d$ with compact closure, and a finite Borel measure $\mu$ on its closure $\overline{\Omega}$.
In Halmos' book it is shown that:
Classical ...

**2**

votes

**1**answer

99 views

### Embedding of weighted sobolev space with exponential weights

In the book by Bensoussan and Lions, they introduce the weighted spaces with exponentially decaying weights to study elliptic equations with bounded coefficients on the whole space $\mathbb{R}^n$. ...

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**0**answers

38 views

### Commuting flows problem for non-Lipschitz vector fields

Let $X$ be a continuous vector field on a (say compact) manifold $M$, if $X$ has ODE uniqueness then we can define its associated flow $\mathcal F_X:\mathbb R\times M\to M$ uniquely given by $\mathcal ...

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**0**answers

21 views

### Regression with incomplete basis [closed]

Suppose that $f(x) = \frac{1}{2}\text{max}(x+10,0) + \frac{1}{3}\text{max}(x+20,0) + \frac{1}{4}\text{max}(x+110,0) + \frac{1}{5}\text{max}(x+120,0)$
If I randomly simulate values for $x$ such that $$...

**3**

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**1**answer

53 views

### How does $E$ closed follow from the upper semicontinuity of the spectrum?

Let $f$ be an analytic function for a domain $D$ of $\mathbb{C}$ into a Banach algebra $A$. Suppose that, for all $\lambda \in D$, $\text{Sp}f(\lambda)$ is finite or a sequence converging to $0$.
...

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**0**answers

139 views

### Does this ideal in $B(L_1)$ have a (bounded) right approximate identity?

I will take a roundabout way to defining this ideal, because (a) this route is how my collaborators and I came to it (b) this alternative definition, rather than the standard one, may suggest a ...

**1**

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**0**answers

48 views

### Global solution of nonlinear Schrödinger equation via blow-up argument

Set $u_0\in H^1 (\mathbb{R} ^N)$ and $1<\alpha < \frac{N+2}{N-2}$.
I want to show that there exists $\varepsilon > 0$ s.t. if $\Vert u _0 \Vert _ {H^1} < \varepsilon$,
then there is ...

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**0**answers

71 views

### Projection of a function $f\in L^1(\Omega)$ onto a finite dimensional subspace

Suppose $\Omega \subset \Bbb R^n$ be a domain such that $|\Omega|<\infty$, $f\in L^1(\Omega)$. Let $Y= \text{span}\{g_1,\dots, g_k\}$.
Is there a characterization of the set of projections of $f$...

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**0**answers

74 views

### Question: can we claim that $W^{1,p}(\Omega) \subset L^1(\Omega) \subset W^{-1,p}(\Omega)$?

Let $\Omega$ be a compact manifold in $\mathbb R^2$. For $1 \leq p \lt
4/3$ can we claim that
$W^{1,p}(\Omega) \subset L^1(\Omega) \subset W^{-1,p}(\Omega)$
with the first inclusion being ...

**2**

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**0**answers

75 views

### Minimizing sequence $\implies$ Palais–Smale sequence

Set $F:\mathbb{R}^n\rightarrow \mathbb{R}$ a $C^2$-function that is bounded from below. Set $x_n$ a minimizing sequence, i.e., $F(x_n)\to \alpha = \inf F$. I want to prove that under the assumption of ...

**2**

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**1**answer

71 views

### Closable unbounded operators and Banach space adjoints

For an unbounded operator $T:\mathcal{H}_1 \to \mathcal{H}_2$, if its adjoint $T^*$ is densely defined, then we know that $T$ is closable. What happens if we replace $\mathcal{H}_1$ or $\mathcal{H}_2$ ...

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**0**answers

25 views

### Showing there exists a solution to a variational inequality

I'm working through a book that provides the following exercise which I'm having trouble with. They cite this paper which I can't find (and even if I could, I can't read italian).
The problem:
Let ...

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**0**answers

46 views

### Is projection method really applicable for numerical solution of linear integral equations in $ L^p \ (p \neq 2)$ setting?

Projection method is a traditional method to numerically handle problem of linear integral equation. The routine way is to do it in $ L^2 $ setting. For example:
Let $ A:L^2(a,b) \to L^2(a,b) $ be a ...

**4**

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**1**answer

157 views

### Does Banach-Mazur distance between regular polygons admit any structure that lends to approximation or exact results in particular situations?

Banach-Mazur distance between $P_5$ and $P_3$ is $d(P_5,P_3)=1+\frac{\sqrt5}2$ where $P_n$ is regular polygon in $n$ sides. Do closed form or approximate results exist (at least at special infinitely ...

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**1**answer

157 views

### Unbounded Fredholms operators

Motivated by the situation of bounded Fredholm operators, I have the following question about "unbounded Fredholm operators".
Let $\mathcal{H}_1$ and $\mathcal{H}_2$ be two Hilbert spaces, and
$$
D: ...

**3**

votes

**2**answers

163 views

### Interior smooth regularity

I recently read the PDE book of L. Evans, and in its chapter 6 some kinds of regularities of second-order elliptic equations were discussed. My question is about its proof of interior smooth ...

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**0**answers

30 views

### A linear first order PDE with boundary condition

I want to solve the following first order PDE
$$
(\star)\quad\begin{cases}
\nabla u\cdot \nabla\xi=f \quad\text{in}\,\Omega, \\
u\mid_{\partial \Omega}=0
\end{cases}
$$
where $\xi\in C^2(\overline{\...

**0**

votes

**1**answer

86 views

### Reference request : How to use Lagrange multiplier technique with infinite (infact uncountably) number of constraints?

I have a constrained maximization problem (maximizing a functional), with number of constraints being uncountable infinite.
It looks something like this. I want to maximize the convex functional $C(f)...

**2**

votes

**3**answers

149 views

### Every linear topological space embeds into the Tychonoff product of linear metric spaces

I need a reference to the following (known?)
Fact. Every topological vector space $X$ over the field of real numbers is topologically isomorphic to a linear subspace of the Tychonoff product of ...

**4**

votes

**1**answer

126 views

### Compute $ \partial_t\int_{\{u(t,\cdot) >0\} } 1\, dx$ in the sense of distributions where $u$ solves a PDE

Let $u:\Omega\subset \mathbb R^N \to \mathbb R$ be bounded function that solves an evolution PDE $\partial_t u(t,x)= L(u(t,\cdot))(x)$, where $L$ is some elliptic operator.
How can I compute the ...