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A Banach space is a complete normed vector space: A vector space equipped with a norm such that every Cauchy sequence converges.
6
votes
Accepted
A Hahn-Banach type extension problem for multiple functionals
If $f_0\ne0$ or if $f_0=0$ and the $f_1,\dotsc,f_n$ are not linearly independent, then the answer is trivial:
In this case there is another functional, say $f_1$, which is in the span of the remaining …
6
votes
Accepted
Radon-Nikodym property in Diestel & Uhl: a definition clarification
Diestel & Uhl can only mean the first interpretation you gave for two reasons:
For the second interpretation, the term “off a fixed set of measure zero” makes no sense.
Even in case $X=\mathbb R$ (wh …
1
vote
How do I apply Brouwer fixed-point theorem in this claim?
Only now I realize the condition that $\zeta$ is nonnegative. (Was it really there in the first formulation of the question?)
With this condition, it is possible to get the required a-priori bound req …
3
votes
On the intersection of two Orlicz spaces
It's many years ago that I read it, but I think that some of the most general interpolation type results for Orlicz spaces were contained in O’Neil, Richard, Integral transforms and tensor products on …
2
votes
Accepted
Definition of a $\psi$-Banach space
The definition makes no sense due to the mixing up of "relatively" (weakly) compact and (not relatively) compact.
I guess that what you mean is:
$\psi$ is strongly-weakly proper on closed balls (that …
1
vote
How do I apply Brouwer fixed-point theorem in this claim?
What is needed is an a-priori $L_\infty$ bound for the solution $v_k$. If you know such an a-priori bound, you can modify $\zeta$ outside of this bound, and you can assume without of generality that $ …
2
votes
Accepted
Example when Kantorovich condition would not hold
Maybe the simplest classical example is a weakly singular kernel
$$K(x,y) = |x-y|^{-\lambda}$$
with some fixed $\lambda\in(0,1)$.
In this example $\int_{\mathbb R^2}K(x,y)^qdx=\infty$ for every $q>0$ …