Functional analysis is the study of infinite-dimensional vector spaces, often with additional structures (inner product, norm, topology), with typical examples given by function spaces. The subject also includes the study of linear and non-linear operators on these spaces, as well as measure, ...

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15 views

Linear Algebra Vector Space and Subspace

If $X$ be an infinite dimensional vector space and $Y$ is subspace of $X$, then show that whether dimension of $Y$ is always finite or infinite also. Also give example of any subspace whose dimension ...
2
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1answer
24 views

Completeness of bounded linear maps

Let $X,Y$ be normed vector spaces over $\mathbb{C}$, and $L(X,Y)$ the space of all bounded linear maps from $X$ to $Y$. Its known that $L(X,Y)$ is a normed(operator norm) vector space. Theorem: ...
1
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1answer
12 views

Proof a real functional is continuous in $C_{[a,b]}$ (verification)

I wish to have some feedback on the following proof of the claim below, either if it is correct, what to fix, or other suggestions. Claim: Let $\psi :[0,1] \times \mathbb{R} \to \mathbb{R}$ be a ...
0
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0answers
13 views

Weak convergence of measures and compact sets

Suppose that we have a sequence of probability measures $\{ \mathbb{P}_n \}$ converging weakly to a probability measure $\mathbb{P}$. Suppose that $M$ is a metric space with a compact subset $K$. I ...
0
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2answers
27 views

Does a bounded sequence in $C^k$ have a convergent subsequence in $C^{k-1}$

Let $K \subset \mathbb{R}^n$ be compact, let $k \in \mathbb{N}$. Let $\{f_n\} \subset C^k(K)$ be a bounded sequence w.r.t $C^k$ norm. Does it have a convergent subsequence in $C^{k-1}(K)$. If so how ...
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0answers
9 views

Space of polynomials as a continuous image of F-space

Let $X=\mathbb{R}[a,b]$. Is there any norm $\|\cdot\|$ on $X$ s.th. $X$ is a continuous image of some $F$-space. ($F$-space means that there exists complete metric s.th. $d(x+z,y+z)=d(x,y)$) ? My ...
0
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0answers
14 views

Weak derivative of a piecewise defined function

I am currently looking at these online notes on PDEs, page 59. How does it follow that if $f^R = \phi(x/R) f(x)$ $ \phi(x) = \left\{\def\arraystretch{1.2}% \begin{array}{@{}c@{\quad}l@{}} 1 ...
1
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1answer
37 views

If $f$ is not continuous then $\ker f$ is dense in $X$

Let $X$ be a normed space and $f:X\rightarrow \mathbb R$ a linear function. I saw an old post with this problem, but there is not a complete proof. For beginning I have to consider that ...
0
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1answer
22 views

Differentiating a function composition

Given $g:R^n \rightarrow R^k$ and $h:R^k \rightarrow R$, we have $f(x) = h(g(x))$. Using the chain rule, we can differentiate $f(x)$ to get $f'(x) = \nabla^Th(g(x))g'(x)$ My question is why do we ...
2
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1answer
41 views

Proving that a function grows faster than another

I'm told to prove or disprove that $4^{\sqrt{n}}$ grows faster than $\sqrt{4^n}$ As n tends to infinity. From my Previous years Calculus I know that if I take the derivative of two functions, and one ...
1
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1answer
12 views

Interior of a set in a normed space

Let $X$ be a normed space, $f \in X'$, $\alpha \in \mathbb{R}$; Prove that: $$\text{int}\{x \in X: f(x) \ge \alpha\}=\{x \in X:f(x)>\alpha \}$$ I noted the set in the left by $A$ and the set in the ...
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0answers
24 views

closed subspace of a linear space

Please, can somebody explain something to me? What exactly does it mean that a subspace $Y\subset X$, where $X$ is a normed space, is closed? It means that every convergent sequence of elements in $Y$ ...
0
votes
0answers
21 views

Matrix Inequality for the identity and a traceless matrix

Given a traceless matrix $C$ $\in M_n(\mathbb{F})$, i.e., tr$(C)=0$, what is the relationship between tr$|\mathbb{I}+C|$ and tr$|C|$? The two matrices are of dimension $n$. This was cross-posted to ...
1
vote
1answer
35 views

Sub-linear functional $f$ satisfies $f(0)=0$

Let $f$ be a sub-linear functional on a real vector space $X$ satisfying $f(x)+f(y)\ge f(x+y);f(\alpha x)=\alpha f(x);\alpha>0$ Show that it satisfies $f(0)=0 $ Thoughts: Now $f(x+y)\leq ...
4
votes
1answer
49 views

Check the proof of $||x||^2$ is not a norm

Show if $f$ is a norm: For $\mathbb{R}^n$, Define $f: \mathbb{R}^n \rightarrow \mathbb{R} $ by $ f(x) = \|x\|^2$ =$\sum_{n} x_n^2 $ I tried to solve if $f$ satisfies the three properties of a norm: ...
3
votes
1answer
26 views

Does a contractible set have contractible preimage, under a linear map?

Let $T:V\to W$ be a linear map of vector spaces, and let $A\subset W$ be contractible. Then is $T^{-1}(A)$ also contractible?
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0answers
51 views

About the gradient of a function in $H^{1}(\Omega)$

let $\Omega \in \mathbb{R}^{n}$ a bounded domain and $u \in H^{1}(\Omega)$ a real function. In the Leoni's Book - A First Course in Sobolev Spaces, the author define $\nabla u = (D_{1} u,\dots, ...
1
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2answers
23 views

About the definition of $L^{\infty}$ norm

Let $\Omega$ a limited domain in $\mathbb{R}^{n}$, the space $L^{\infty}(\Omega)=\{f: \Omega\to\mathbb{R} $ measurable $; ||f||_{L^{\infty}(\Omega)}<\infty\}$. Then if a function $f \in ...
2
votes
2answers
35 views

Prove norm inequality

It is given that $$\left\lVert x-y\right\rVert =\left\lVert y-z\right\rVert = \left\lVert z-x\right\rVert \qquad (1) $$ where $x,y,z \in \Bbb R^2$ and $ \left\lVert x\right\rVert=\sqrt ...
1
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0answers
22 views

Inequality in Banach space [duplicate]

So I have to either prove or disprove this inequality: $$ \left\lVert x\right\rVert^2 - \left\lVert y\right\rVert^2 \le \left\lVert x-y\right\rVert \left\lVert x+y\right\rVert$$ I know this to be ...
0
votes
1answer
13 views

problem on annihilators on finite dimensional spaces

Suppose $V$ and $W$ are subspaces of a finite-dimensional vector space $U$. Show that if $V^0 \subset W^0$ then $W \subset V$ This is an exercise problem in Linear Algebra Done Right, 3rd ...
3
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2answers
32 views

Composition involving bounded linear operators

I recently come across the following statement mentioned in a proof: Let $X,Y$ be normed linear spaces and $T:X \rightarrow Y$ be a linear operator. if for every bounded linear functional $U: Y ...
0
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0answers
26 views

how to prove this function has zeros interlacing and including those of Riemann zeta

Let $\chi (t) = \dfrac{4 i \pi \zeta (t) \left( \left( \ddot{\Psi} \left( \frac{t}{2} \right) - \ddot{\Psi} \left( \frac{1}{2} - \frac{t}{2} \right) \right) \zeta (t)^3 - 48 \zeta (t) \dot{\zeta} (t) ...
0
votes
0answers
25 views

Convex function from condition

Can we deduce that $F$ is a convex function (i.e $F''(t)>0, \forall t>0$) from the following conditions: $F(t)=\int_{0}^t f(\xi) d\xi$, $0\leq \theta F(t)< t f(t), \forall t>0$ The ...
1
vote
3answers
40 views

application of the inequality $\|fg\|_1 \leq \|f\|_p\|g\|_q$

application of the inequality $\|fg\|_1 \leq \|f\|_p\|g\|_q$ where $1/p + 1/q = 1$ I know this is a straight application of the inequality, but how am I assured that the integral of ...
0
votes
1answer
9 views

infimum of operator norms of iterations of linear operators

I am currently reading a proof in which a fact is used without proof: For a Banach space $X$ and a bounded linear operator $T: X \to X$, $$ \lim_{n \to \infty} \| T^n \|^{\frac{1}{n}} = \inf_{n ...
0
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1answer
26 views

When a set is convex, how does the polar set of its polar set equal the original?

I have read the following proposition, and haven't been able to connect the convexity of $X$ to the statement's main equality. Any guidance would be much appreciated. Define $X^\text{o}$, the polar ...
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0answers
18 views

Prove dimension finiteness for a separable subspace of $L^\infty(0,1)$. [on hold]

Let $X$ be a separable subspace of $L^\infty(0,1)$. How do I prove that $X$ is finite dimensional? Thanks in advance.
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votes
1answer
11 views

Unique solution of an integral equation in $L^1[0,1]$

Let $h\in L^1[0,1]$. Prove that there is a unique solution (almost everywhere) of the following integral equation: $$f(x)=h(x)+\frac{1}{2}\int_0^x\log(1+f(y)^2)dy$$ The idea is to use the fixed-point ...
0
votes
0answers
19 views

Integral inequality $L^p$ spaces

I'm trying to solve this problem: Let $1<p<\infty$. Then let $f:(0,\infty)\to [0,\infty]$ a measurable non negative function. It's true the following inequality: $$\int_0^\infty ( ...
1
vote
1answer
70 views

$\lim_{n \to \infty }\int_{0}^{n}\frac{n \cdot e^{\frac{x}{n}}}{x^4+n^2}dx=$?

$$\lim_{n \to \infty }\int_{0}^{n}\frac{n \cdot e^{\frac{x}{n}}}{x^4+n^2}dx=?$$ I am allowed to used all the classical techniques of calculus, and this was a question from measure theory when we were ...
6
votes
2answers
74 views

$\lim_{n \to \infty} \int_{0}^{n}(1-\frac{3x}{n})^ne^{\frac{x}{2}}dx$=?

$$\lim_{n \to \infty} \int_{0}^{n}\left(1-\frac{3x}{n}\right)^ne^{\frac{x}{2}}dx$$ I thought about using the theorem of monotonic convergence and had ...
0
votes
1answer
34 views

Uniqueness of a linear operator

The wikipedia entry for bounded operators shows that for the space $X$ of trigonometric polynomials on $[-\pi,\pi]$ with norm $$\lVert P\rVert = \int_{-\pi}^{\pi}\lvert P(x)\rvert ...
0
votes
1answer
19 views

Minimum of the Schatten 1-norm

Given two operators or non-zero matrices $A$ and $B$, where $A\neq B$, tr$(A)=1$ and tr$(B)=1$ and tr$(A-B)=0$, what is a lower bound of the Schatten p-norm ($p=1$) $\|A-B\|_1$? Any helpful ...
0
votes
0answers
11 views

A linear functional is continuous iff its kernel is closed [duplicate]

A linear functional defined on a normed space is continuous if and only if its kernel is closed.
1
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1answer
14 views

Tempered representatives of a special class of distributions

Suppose that a distribution $R\in D'(\Bbb R)$ satisfies the following estimation for an independent constant $c$: $$\forall \phi\in D(\Bbb R)\quad |\langle R,\phi\rangle|\le c\|\phi, \,L^1(\Bbb ...
0
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1answer
10 views

How to turn convergence in probability a statement involving n?

Def: for every $\epsilon$ $\lim_{n}P(|X_n-X|>\epsilon)=0$ How to turn it into a statement of there is an N s.t. n>N... Shall we make it $P(\lim_{n}|X_n-X|>\epsilon)=0$ first?
0
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0answers
12 views

Proof for multi-dimensional contraction operator

I have the following two dimensional operator and I need to prove $\Psi(\boldsymbol{u})$ is a contractor. $\Psi(\boldsymbol{u})_1(t) \equiv \hat{a}_1(t) + \int_{0}^{t}u_1(t-y)p_{1,1}(t-y) ...
0
votes
1answer
27 views

Sobolev space interpolation

We are attempting to show that, for $t_0,t_1\in\mathbb{R}$, and $t_\sigma=(1-\sigma)t_0+\sigma t_1$ for any $\sigma\in[0,1]$, then the following inequality holds for any $u\in H^{t_1}(\mathbb{R}^d)$: ...
0
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0answers
32 views

example of positive operators a,b, $a\le b$ but $b^2-a^2$ is not positive [on hold]

Give an example of a C*-algebra $\mathscr{A}$ and positive elements a,b in $\mathscr{A}$ such that $a\le b$ but $b^2-a^2\notin \mathscr{A}_+$, i.e. $b^2-a^2$ is not positive element in $\mathscr{A} $
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0answers
44 views

Topology in space of test functions $\mathcal{D}(\Omega)$ and space of distributions $\mathcal{D}'(\Omega)$

We can concluded that $\mathcal{D}(\Omega):=\bigcup_{K \in \mathcal{K}(\Omega)} \mathcal{D}_K(\Omega)$ (where $\mathcal{K}(\Omega)$ denotes the union of all compacts set content in a open subset ...
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2answers
30 views

Further generalising Holder's inequality

I have proved the following theorem in an earlier part of the question: Let $p,q \geq 1$ be such that $\frac{1}{p} + \frac{1}{q} = 1$. Show that: $$\|fg\|_1 \leq \|f\|_p \|g\|_q$$. I proved this ...
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0answers
16 views

a space has empty algebraic interior

Can someone help me to continue? Let $p \in [1,\infty)$, $l_{p}^{+}=\{((x_{n})_{n \ge 1}) \in l_{p}|x_{n}\ge 0, \forall n\ge 1 \}$. I am trying so show that $l_{p}^{+}$ has empty algebraic interior. ...
2
votes
1answer
40 views

What is “white noise” and how is it related to the Brownian motion?

In the Chapter 1.2 of Stochastic Partial Differential Equations: An Introduction by Wei Liu and Michael Röckner, the authors introduce stochastic partial differential equations by considering ...
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0answers
32 views

Help proving a map between Sobolev spaces is continuous [duplicate]

We are trying to show that the map $f\mapsto|f|$ is a continuous (nonlinear) map from $W^{1,p}(\Omega)\to W^{1,p}(\Omega)$ for any bounded/open region $\Omega$ and for $p\in[1,\infty)$. We have tried ...
2
votes
4answers
41 views

Compact operator in Hilbert spaces reach the maximum in the sphere.

I found the following question in my textbook: (QUESTION) Let $\mathcal{H}$ a Hilbert space and $T: \mathcal{H} \rightarrow \mathcal{H}$ a compact operator. Show that exists $x \neq 0$ in ...
1
vote
1answer
37 views

Is there a nice way to express $\psi_1$ using this orthonormal sequence?

Suppose that $H$ is a separable Hilbert space and $(\psi_n)_{n=1}^{\infty}$ is a complete orthonormal sequence in $H$. We define a sequence $(\phi_n)_{n=1}^{\infty}$ by $$ \phi_n=\psi_1+\psi_{n+1}\ ;\ ...
0
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0answers
30 views

Is $C_0(\mathbb{R})$ a Banach space?

Let $C(\mathbb{R})$ be a Banach space of continuous , real functions defined on $\mathbb{R}$, with supremum norm, and let $C_0(R)$ be the subspace of functions vanishing at infinity. Is ...
1
vote
2answers
117 views

Weak topology and strong topology in a Banach space.

I have a doubt about weak topology in a Banach space. Let $\mathcal{B}$ a infinite dimensional Banach space, I understood that the weak topology in $\mathcal{B}$, is the topology generated by $\Sigma ...
1
vote
1answer
11 views

affine hull of a sum of sets

Could someone, please, help me with a inclusion?I am trying to prove that: $$Aff(A+B)=Aff(A)+Aff(B)$$, where $A,B \subset X$, $X$ a linear space; I proved that $Aff(A+B)\subset Aff(A)+Aff(B)$,but I am ...