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Special functions, orthogonal polynomials, harmonic analysis, ordinary differential equations (ODE's), differential relations, calculus of variations, approximations, expansions, asymptotics.

5 votes
Accepted

In what ways is the standard Fourier basis optimal?

Here's something similar to what you conjecture. Let's work instead with functions $\mathbb{R} / 2\pi \mathbb{Z} \to \mathbb{C}$; one can do similar things in the real-valued case but I find the comp …
Mark Meckes's user avatar
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3 votes

Is anything known about $w^*(x)=\sup_y w(x+y)/w(y)$ for measurable functions w on $R^n$

This is a long comment rather than an answer. In this paper of Klartag and Milman, the following operation on functions is defined and called the Asplund sum (as far as I can tell, that name has not …
Mark Meckes's user avatar
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8 votes
3 answers
3k views

Convergence of orthogonal polynomial expansions

"Everyone" knows that for a general $f\in L^2[0,1]$, the Fourier series of $f$ converges to $f$ in the $L^2$ norm but not necessarily in most other senses one might be interested in; but if $f$ is rea …
Mark Meckes's user avatar
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1 vote

l^p space inequality related to compressed sensing

Regarding the last part of the question, I haven't looked at either of the following books myself, but I've seen them referred to for systematic presentations of the theory of quasinormed spaces (whic …
Mark Meckes's user avatar
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11 votes

Almost-converses to the AM-GM inequality

It's not precisely what you asked about, but this paper by Gluskin and Milman shows that, for "most" sequences $a_1, \dotsc, a_n$, the AM-GM inequality can be reversed up to a multiplicative constant. …
Mark Meckes's user avatar
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16 votes
Accepted

Stronger version of the isoperimetric inequality

A classical result along these lines is Bonnesen's inequality, which states $$ L^2 - 4\pi A \ge \pi^2 (r_{out} - r_{in})^2, $$ where $L$ is the length and $A$ is the enclosed area of a simple planar c …
Mark Meckes's user avatar
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3 votes

Approximate a probability distribution by moment matching

In the context of 3), what I have heard from folklore is that when (1) holds, the Kolmogorov distance (not total variation) is bounded by something like $1/\sqrt{m}$. This bound follows if (1) holds …
Mark Meckes's user avatar
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