## New answers tagged banach-spaces

4
votes

### When a quasinilpotent is nilpotent?

I quasi-nilpotent operator $T \in B(X)$ is nilpotent if and only if $0$ is a pole of its resolvent $(\cdot - T)^{-1}$, which means that there exists a number $M \ge 0$ and an integer $n \ge 1$ such ...

2
votes

### When a quasinilpotent is nilpotent?

This is almost tautological, but suppose that the quasi-nilpotent operator $T$ has the property that, for some $p\in\mathbb N_+$, the equality $\|T^{np}\|=\|T^p\|^n$ holds for infinitely many $n\in\...

4
votes

Accepted

### Why do distributional isomorphisms preserve joint distribution?

I cannot resist to provide an easily citable reference to Rudin's beautiful paper $L_p$-Isometries and equimeasurability, where Rudin proves the much stronger result that if $p$ is not an even integer ...

5
votes

### An example of non-invertible operator $F$ such that $P_nF$ is invertible on $\operatorname{Im}P_n$ or proving that It is impossible

(In the following I assume that the word "invertible" in the question means "bijective".)
Your assumptions do not imply that $F$ is bijective (however, they imply that $F$ is ...

2
votes

### When do the weak-star and compact convergence (compact-open) topology coincide on the dual of a Banach space?

Weak$^*$-convergence of a net means pointwise convergence on singletons, and therefore (uniform) convergence finite sets. Uniform convergence on compact subsets means exactly that. Since finite sets ...

2
votes

Accepted

### Computation of tangent functional

$\newcommand{\om}{\omega}\newcommand{\Om}{\Omega} \newcommand{\de}{\delta}\renewcommand{\th}{\theta}$For each real $t>0$ and some $\om_t\in\Om$,
\begin{equation*}
\|x+ty\|
=|(x+ty)(\om_t)|=|...

2
votes

### Computation of tangent functional

The idea is that, for $|x(\omega)|\sim 1$ and $t$ small, $$|x(\omega)+ty(\omega)|\sim|x(\omega)^2+tx(\omega)y(\omega)|\sim 1+tx(\omega)y(\omega)$$
Indeed, if $|x(\omega)|<1-\varepsilon$, then for ...

2
votes

Accepted

### Gateaux differentiability of the norm in Banach spaces

We have the following definitions:
\begin{equation*}
V^*:=\{f\in S^*\colon\exists x\in S\ f(x)=1\},
\end{equation*}
where $S$ and $S^*$ are the unit spheres in the underlying Banach space $E$ and ...

2
votes

Accepted

### Potentially elementary question on affine functions on Banach spaces

A function $f$ from a linear (or, more generally, affine) space $A$ is affine (see e.g. this post) if for all $a$ and $b$ in $A$ and all real $t$ one has $f((1-t)a+tb)=(1-t)f(a)+tf(b)$. If $f$ is ...

4
votes

Accepted

### Approximating continuous functions from $K\times L$ into $[0,1]$

The following provides a construction of $g_i, h_i$ satisfying the required conditions (the ranges of these functions can even be in $[0, 1]$):
Claim: For each $k \in K$, there exists an open ...

0
votes

### Does weak-* convergence in $W^{1,\infty}$ imply weak-* convergence in $L^\infty$?

I was wondering about the same question and was surprised that I could not find an answer to this anywhere. The answer to your question is yes.
Let $(X, \|\cdot\|)$ be a normed vector space and $Y \...

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