A vector space $E$, generally over the field $\mathbb R$ or $\mathbb C$ with a map $\lVert \cdot\rVert\colon E\to \mathbb R_+$ satisfying some conditions.

learn more… | top users | synonyms

2
votes
0answers
63 views

Space of Continuous mappings to metric spaces

I want to ask whether some basic result from the space $C([0,1],R)$, where $R$ is the real space carries over to the space $C([0,1],E)$, where $(E,\|\cdot\|_E)$ is a metric space. We know that ...
2
votes
1answer
150 views

If the dual spaces are isometrically isomorphic are the spaces isomorphic?

Let $X$, $Y$ be Banach spaces such that the duals $X^\ast$ and $Y^\ast$ are isometrically isomorphic. Are $X$ and $Y$ necessarily isomorphic? The answer to the question whether $X$ and $Y$ are ...
2
votes
1answer
80 views

The density of diagonalizable matrices of $M_n(\mathbb{C})$ problem.

For any matrix $A = (a_{ij})_{1\leq i,j\leq n} \in M_n(\mathbb{C})$, we pose $||A|| = \max_{1\leq i,j\leq n} |a_{ij}|$. $1.$ Show that $||.||$ define a norm on $M_n(\mathbb{C})$ and that $\forall A, ...
0
votes
1answer
40 views

How to detect reflexivity of the closure

Consider the space of continuous bounded functions on a bounded interval. Its closure for the Lebesgue $L_p$ norm is reflexive when $1 < p < \infty$, but it is not reflexive for $p = 1$. How ...
1
vote
3answers
202 views

Sum of closed subspaces of normed linear space

Problem Suppose $R$ is a normed linear space, then show that: If $M$ is closed subspace of $R$ and $N$ a finite dimensional subspace of $R$, then the set $$M+N=\{ z : z = x + y , x \in M , y \in N ...
1
vote
1answer
63 views

Preannihilator of the image of an adjoint of a bounded operator

Let $E,F$ be normed spaces and $F\colon E\rightarrow F$ be a linear bounded operator. Denote by $$A'\colon F'\rightarrow E'$$ the adjoint of the operator between the topological duals of the normed ...
1
vote
0answers
50 views

Complete inner product space always has an orthonormal basis

I am trying to understand inner product space. My text book says: A complete linear space with scalar product(not necessarily seperable) always has an orthonormal basis. I looked up the ...
0
votes
1answer
31 views

Check whether a sequence belongs to an open ball

How to check if the sequence x=( x1 , x2 ,...) where xn =1-(1/n) belongs to the open ball B(0,1) in the normed space l^∞ of all bounded sequences with the norm defined by ...
1
vote
2answers
137 views

Collecting things that are preserved by (isometric) isomorphisms between normed spaces

I would like to collect a list of things that are preserved under isomorphisms and isometric isomorphisms. The reason is that I hope to get a better perception of their importance in Functional ...
0
votes
1answer
218 views

Unit balls in normed spaces.

Assume we talk about the $n$ dimensional vector space over the reals. It is easy to see that for any norm the unit ball is a convex symmetric set. And here is my question : Let $A$ be a bounded , ...
2
votes
1answer
72 views

Convergence in Sobolev Spaces

Consider the bounded mapping $A:W^{1,p}(\Omega) \rightarrow W^{1,p}(\Omega)^{*}$ where $A$ is defined as: $\langle A(u),v \rangle\text{ } := \int_{\Omega}a(x,u,\nabla u)\cdot \nabla v + c(x,u,\nabla ...
4
votes
1answer
110 views

Euclidean space without orthonormal basis

I've been thinking about: Problem Give an example of a nonseperable Euclidean space which has no orthonormal basis. My Argument I know if a Euclidean space $R$ has at most countable basis, ...
0
votes
0answers
86 views

Absolutely convergent series in normed linear space

I want to prove that in a normed linear space $X$ if for all absolutely convergent series $\sum\limits^{\infty}_{n=1}x_n$, the series $\sum\limits^{\infty}_{n=1}T(x_n)$ is convergent, then $T:X\to Y$ ...
2
votes
1answer
62 views

Algebraic dimension of infinite-dimensional Banach Space

I am trying to show: Algebraic dimension of infinite-dimensional Banach Space is uncountable. By algebraic dimension it is meant that the cardinality of the Hamel Basis of the space. Suppose ...
1
vote
0answers
82 views

Every inner product space is a normed space which is also a metric space

As I am pretty sure that everybody knows that a Hilbert space is a space that is a complete, separable and generally infinite dimensional inner product space. By the means of completion, every Cauchy ...
2
votes
2answers
79 views

restriction a non compact operator to compact operator

If $T\in\mathcal{B}(X,Y)$ is not compact can the restriction of $T$ to an infinite dimensional subspace of $X$ be compact?
1
vote
3answers
101 views

Is there a norm on ${\Bbb R}^{\Bbb N}$

Let $E={\Bbb R}^{\Bbb N}$ be the real vector space of real sequences. 1) Is there a norm on $E$? 2) Is there a norm $N$ on $E$ such that the restriction of $N$ to $\ell^2$ is finer than the ...
3
votes
1answer
139 views

Weak continuity in Sobolev Spaces

First consider the following two Sobolev Embedding Theorems. Theorem 1: The continuous embedding $W^{1,p}(\Omega) \subset L^{p^{*}}(\Omega)$ holds provided the exponent $p^{*}$ is defined as ...
0
votes
1answer
264 views

Norm of Matrix transpose

I have a problem below: Let $\|\cdot\|$ denotes the norm matrix \begin{equation} \|A\|=\max \frac {\|Ax\|}{\|x\|}, \end{equation} for every $A$. Now suppose that $H: \mathbb{R}^k \rightarrow ...
1
vote
1answer
128 views

Closure of $B=\{ f\in C'[0,1] : |f(x)|\leq 1, |f'(t)|\leq 1 \forall t\in[0,1]\}$ (NBHM $2005$)

If $A$ is the closure in $\mathcal{C}[0,1]$ of the set $B$ where $$B=\{ f\in C'[0,1] : |f(x)|\leq 1, |f'(t)|\leq 1 \forall t\in[0,1]\}$$ Then Which of the following is true? $A$ is closed. $A$ ...
0
votes
1answer
27 views

Bounded & Norm space [closed]

Can someone help me on this exercise ? Thanks!
6
votes
1answer
169 views

Isomorphisms between Normed Spaces

Between two normed (linear) spaces there are several notions of isomorphisms: Linear isomorphisms: linear bijective maps Topological isomorphisms: linear homeomorphisms (due to the linearity these ...
1
vote
1answer
50 views

About the closedness of Banach space

I'm reading a proof in trying to prove that a Banach space $X$ is reflexive if and only if $X^{*}$ is reflexive. There's a point in the proof saying that $X$ is a closed subspace of $X^{**}$, but I ...
2
votes
0answers
84 views

Linear Operators: Continuous $\Rightarrow$ Bounded

Let $T:V\rightarrow V'$ be a continuous linear operator between two normed vector spaces $V,V'$. Show that it is bounded. Continuity is defined as $\lim_{n}\|x_n-x\|=0\Rightarrow ...
0
votes
2answers
131 views

Are the two infima equal?

Let $R$ be a ring (not necessarily commutative) or an algebra over some field with a norm defined on it and let $I$ be an ideal in $R$. Let $a,b \in R$. Does it hold that $\inf_{i,j \in I}\|ab + ai ...
1
vote
3answers
107 views

If every inner product space can be converted into a norm space, then why is there a distinction between the two?

If every inner product space can be converted into a norm space, then why is there a distinction between the two? $$\|x\| = \sqrt{\langle x,x\rangle }$$
2
votes
0answers
87 views

normed space functional analysis [closed]

applying this lemma for e1=(1,0,0) e2=(0,1,0) e3=(0,0,1) what is maximum of c? i found c=1 am i right?
3
votes
2answers
102 views

Completeness/Compactness of a subset in a normed linear space

Let $(X,\|\cdot\|)$ be the normed linear space consisting of the sequences $a=(a_n)_{n=1}^{\infty}$, for which the corresponding series $\sum_{n=1}^{\infty} a_n$ converges absolutely, with norm ...
2
votes
1answer
55 views

Endpoint-average inequality for a line segment in a normed space

Let $X$ be a normed vector space over $\mathbb R$. What is the smallest universal constant $C>0$ such that the inequality $$\|x\|\le C\int_0^1 \|x+tv\|\,dt\tag{1}$$ holds for all $x,v\in X$? ...
2
votes
1answer
57 views

The proof of the triangle inequality of this norm

Let $V$ be a normed vector space and $W$ a closed subspace of $V$. It is possible to define a norm on $V/W$ (the quotient space) by defining $\|v + W \|_{V/W} = \inf_{w \in W} \|v + w\|$. As an ...
1
vote
1answer
34 views

Proof of uniqueness of zero for this norm

Let $V$ be a normed vector space and $W$ a closed subspace of $V$. It is possible to define a norm on $V/W$ (the quotient space) by defining $\|v + W \|_{V/W} = \inf_{w \in W} \|v + w\|$. As an ...
1
vote
0answers
33 views

Proof about scalar multiplication of this norm

Let $V$ be a normed vector space and $W$ a closed subspace of $V$. It is possible to define a norm on $V/W$ (the quotient space) by defining $\|v + W \| = \inf_{w \in W} \|v + w\|$. As an exercise I ...
4
votes
3answers
165 views

Continuous Linear Functional on $\ell^{\infty}$

I'd like help answering two questions. 1) Prove that there is a continuous linear functional on $\ell^\infty$ such that $f(e_n)=0 \ \forall n \in \Bbb{N}$ and $f(a)=5$ where $a=(1,1,1,1,1,1,\ldots)$. ...
3
votes
0answers
65 views

why is test function space $\mathcal{A}$ complete

I am trying to find out, why the space $$\mathcal{A}:=\left\{\phi\in C_0(\mathbb{R}^{2d})|\;\|\phi\|_\mathcal{A}:=\int_{\mathbb{R}^d}\sup_{x\in\mathbb{R}^d}|(\mathcal{F}_p\phi)(x,y)|\;\mathrm ...
2
votes
2answers
97 views

Is $C[a,b]$ a closed linear subspace of $L^{p}(a,b)$

I am not sure about the last step of my proof: $(L^{p}(X,A,\mu), \|\cdot \|)$ is a normed $L^{p}$ space of p-integrable functions. $L^{p}(a,b)$ is the space of p-integrable functions on (a,b). ...
2
votes
1answer
74 views

Find the norm of $A$ where $(Af)(t)=tf(t)$

I have the following problem that I would like to ask you about: I have $X$ as my normed linear vector space and $B(X,X)=B(X)$ as my space of all operators $A: X \to X$, where for all $A \in B(X)$ is ...
1
vote
2answers
36 views

Another question about integrable functions with a transform

I am an engineering student, and taking a real analysis course at demand of my advisor, my inexperience in proofs is giving me hard time. I stumbled upon this example, whose proof left as an exercise. ...
1
vote
2answers
39 views

Negative exponential distance

Let $X := \left\{(a_k)_{k \in \mathbb N}, a_k \in \mathbb C\right\}$. Let $d\left( (a_k)_{k \in \mathbb N}, (b_k)_{k \in \mathbb N} \right) := e^{-u}$ with $u$ the smallest integer $k$ such that $a_k ...
2
votes
1answer
56 views

Differentiability of function defined as integral form

Let $H(t)=\int_{\Bbb R}|f(x)+tg(x)|^p\mathrm dx$ and $f,g\in L^p(\Bbb R)$. Then, how to prove that $H$ is differentiable and find its derivative? I think it's impossible to find it by ...
0
votes
0answers
19 views

Limit of $L^r$ norm in lebesgue measure theory [duplicate]

Let $f\in L^r$ for some $r>0$ and $\mu (X)=1$. Then, prove that $\lim_{p\to 0}||f||_p=\exp(\int \log|f|d\mu)$. This is from chapter $L^p$ spaces, but I don't have any idea. How to make $\log$? ...
2
votes
1answer
204 views

Are functions in Lp space always bounded?

I know that functions in $L^2$ space have finite norms by definition, but are they also bounded "almost everywhere" ? So say for instance the following functions norm is finite but it is not bounded. ...
2
votes
1answer
108 views

About open mapping and closed range theorem

I'm self-learning Functional Analysis in Rudin's book and found some following statement hard to understand. Hope someone can help me clarify this. 1) $X, Y$ are Banach spaces, $T \in B(X,Y)$, let ...
3
votes
2answers
42 views

If a function f(x) has a p-norm then does it automatically have a (p-1)-norm ? or a (<p)-norm

Hi Id like to know if a function has a p norm does that mean it automatically has a norm for al llower values of p ?. What about higher values ?. I am trying to write a proof and would like to use ...
0
votes
1answer
75 views

Properties of subspaces of Normed Vector Spaces

How does it follow that a subset of a normed vector space cannot be open if it does not contain an open ball $B_{\epsilon}(0)$ where $\epsilon > 0$? I just want to confirm also that for normed ...
5
votes
1answer
73 views

Prove, that f is a linear map.

$U,V$ - Euclidean spaces $f:U \rightarrow V$ $f(0)=0$ $ \forall _{u,v \in U}:d(f(u),f(v))=d(u,v)$ Prove that $f$ is a linear map. I'm thinking about something like this: $||f(u+v)|| =d(f(u+v),0) = ...
4
votes
2answers
118 views

Is there always an injective map from a space in its dual space?

Today our teacher said that dual spaces are "big" and told us that this is a consequence by Hahn-Banach's theorem. So I was wondering whether the dual space of a space is always "bigger" or equal ...
2
votes
2answers
200 views

Any two norms on finite dimensional space are equivalent

Any two norms on a finite dimensional linear space are equivalent. Suppose not, and that $||\cdot||$ is a norm such that for any other norm $||\cdot||'$ and any constant $C$, $C||x||'<||x||$ for ...
3
votes
1answer
98 views

Subspaces of a Topological Vector Spaces

I have a few questions about topological spaces which I am currently studying. First some definitions that I am using: Definition of subspace topology: Given a topological space $(X,\tau)$ and a ...
2
votes
2answers
65 views

Functional analysis, help to show a short result

The following problem is from the theory of compact operators: Suppose $X,Y$ are normed spaces and $T:X\to Y$ is linear. Show that if $T$ is compact and invertible then $\mbox{dim}(X)$ and ...
0
votes
2answers
358 views

Show these two norms are not equivalent?

I have the following two norms on $C[a,b]$ : $$||x||_1= \int_a^b |x(t)|dt$$ $$||x||_\infty = sup_{t \in [a,b]}|x(t)|$$ $\forall x \in C[a,b]$. I need to prove that these are not equivalent. They are ...