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

24 votes

Which functions of one variable are derivatives ?

I can't claim much knowledge here, but I am given to understand that the class of differentiable functions (or the class of functions which are derivatives of such) is really quite nasty and complicat …
Todd Trimble's user avatar
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56 votes
Accepted

Do these properties characterize differentiation?

Yeah, these force it to be ordinary differentiation. We have to show that for each fixed $x_0 \in \mathbb{R}$, the composite $$C^\infty(\mathbb{R}) \stackrel{L}{\to} C^\infty(\mathbb{R}) \stackrel{e …
Todd Trimble's user avatar
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13 votes
Accepted

Functions whose antiderivative behaves like xf(x)

Note: This is a major rewrite of my earlier answer, to include necessary and sufficient conditions applicable to an even wider class of functions. Instead of expanding to the class of all analytic f …
Todd Trimble's user avatar
  • 53.3k
9 votes

How does the function g(x) behave as x tends to 1?

This appears as Problem 4 of an MSRI Emissary publication, and indeed this problem has been discussed by Noam Elkies. See also his web page (problem 8 here), where he links to a solution. The conclu …
Todd Trimble's user avatar
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4 votes
Accepted

Who first found this characterization of Lebesgue integration?

I don't know if this is of help, but I have seen this idea for defining integration elsewhere, specifically on pages 10-11 of Reed and Simon's Functional Analysis. It would go something like this: let …
Todd Trimble's user avatar
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13 votes
Accepted

Connectifications?

After seeing wood's last comment (comment #2 under his question), I've decided to add a few words (a bit too many for a comment) which hopefully make clear the force of Qiaochu's answer. Generally s …
Todd Trimble's user avatar
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53 votes

Why is differentiating mechanics and integration art?

I hesitate to answer again, but I agree with comments by Deane Yang and others that so far the discussions haven't quite gotten to the bottom of things. (Not that I promise to succeed in doing so now, …
Todd Trimble's user avatar
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23 votes
1 answer
2k views

Monotone functions are differentiable a.e. and Hilbert's Fifth Problem: what's the connection?

In Andrew Gleason's interview for More Mathematical People, there is the following exchange concerning Gleason's work on Hilbert's fifth problem on whether every locally Euclidean topological group is …
Todd Trimble's user avatar
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146 votes
Accepted

Why is differentiating mechanics and integration art?

One relevant thing here is that you are referring to differentiating and integrating within the class of so-called elementary functions, which are built recursively from polynomials and complex expone …
Todd Trimble's user avatar
  • 53.3k
4 votes

Solvability in differential Galois theory

It's probably not quite what you're looking for, but a necessary and sufficient condition for existence of elementary antiderivatives is given in this nice write-up on Dave Rusin's web, here. He inclu …
Todd Trimble's user avatar
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8 votes

Variable-centric logical foundation of calculus

In one of my comments over at the other thread (this other thread), I had mentioned some discussion at the $n$-Category Café about a differential $\lambda$-calculus (from the looks of it, different to …
Todd Trimble's user avatar
  • 53.3k
7 votes

Linear independence of exponential functions: a reference

I will recount the more general statement of linear independence of characters, given in Lang's Algebra book, and credited to Artin. Let $G$ be a group, and $K$ a field. Then distinct homomorphisms $\ …
Todd Trimble's user avatar
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7 votes
Accepted

$\sum_{k=1}^n\frac{\sin kx}{k^\alpha} >0\quad\text{for all}\ n=1,2,3,\ldots\ \text{and}\ 0<x...

Comment by Cherng-tiao Perng converted to an answer: It appears that Theorem A of this paper solves your problem.