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

3 votes

Are there any techniques for solving a differential equation of the form $f ' (x) = f( f( x ...

For what I know, the standard method is the Taylor series expansion at a fixed point, i.e. at a point $x=a$ such that $f(a)=a$.
Alex Gavrilov's user avatar
1 vote

asymptotic with a very degenerate stationary phase

(I am not really an expert, so it should be taken with a pinch of salt.) One may take $f=1$ and simply compute the integral using polar coordinates. (There are minor problems with convergence, but …
Alex Gavrilov's user avatar
6 votes

Lindelöf hypothesis claim

(Not an answer of any sort, just too long for a comment.) The main result seems to be an integral equation (1.3) of the form $$\int_{-\infty}^\infty K(t,\tau) |\zeta(\tfrac{1}{2}+it\tau)|^2\,d\tau= …
Alex Gavrilov's user avatar
2 votes

Closed-Form solution for system of simple nonlinear equations

(Not really an answer, but hopefully may be helpful.) It is convenient to consider this as a problem of finding a solution of the equation $$(N-2)x-\sum_i\delta_i\sqrt{x^2-d_i}=0,$$ for a given vect …
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
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