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

4 votes

Analytic approximations of smooth vector fields

I believe the most natural approach to this particular question is via Fourier analysis. In the periodic case we have the series $$u(x)=\sum_{k\in\mathbb{Z}^3}u_k e^{2\pi i (k,x)},$$ and the condition …
Alex Gavrilov's user avatar
24 votes
2 answers
2k views

Is the Invariant Subspace Problem arithmetic?

Invariant Subspace Conjecture: A bounded operator on a separable Hilbert space has a non-trivial closed invariant subspace. Can this conjecture be reformulated as an arithmetic statement, that is, $ …
1 vote
Accepted

Is this simple-looking weighted poincare-sobolev inequality correct

This inequality is not true. (For $2<p<2^*$, that is. This is how I understood the question.) To see why, imagine that the function $f$ has a huge peak in the area where the weight $w$ is very smal …
Alex Gavrilov's user avatar
13 votes
Accepted

History of publication of von Neumann's characterization of orthogonally invariant matrix norms

Probably there is no way to know it for certain, but it's a safe bet that this was related to Fritz Noether https://en.wikipedia.org/wiki/Fritz_Noether . He was a brother of Emmy Noether and also a …
Alex Gavrilov's user avatar
2 votes

Complex structure on $L^2(\mathbb R)$ generalizing the Hilbert transform

EDIT: This solution does not satisfy the third condition, which rules out the Hilbert transform itself. So, this is an answer to different question. I do not delete it in hope it may be useful for so …
Alex Gavrilov's user avatar