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

29 votes

Are there any nonlinear solutions to $f(x+1) - f(x) = f'(x)$?

Yes, there exist nonlinear solutions. Multiplying by $e^{x+1}$ and setting $g(x):=e^x f(x)$ transforms the question into finding a solution to $g(x+1)=eg'(x)$ not of the form $e^x(ax+b)$. Start with …
Bjorn Poonen's user avatar
  • 23.8k
21 votes
Accepted

A two-variable Fourier series and a strange integral

Here is a proof of the conjecture, a proof that also shows how to compute the integrals explicitly. The proof is somewhat similar to David Speyer's approach, but instead of using multivariable residu …
Bjorn Poonen's user avatar
  • 23.8k
8 votes

Asymptotic series for roots of polynomials

This is the well-known theory of Puiseux series (substitute $T=1/n$ to match the notation in that website). The convergence of the series is mentioned there too. So in your dissertation, I would rec …
Bjorn Poonen's user avatar
  • 23.8k
4 votes
Accepted

Can I detect the point of impact without looking at it?

Andrew's comments showed me that in my first answer I was misunderstanding several aspects of his question. Since I am still not entirely sure that I am capturing the spirit of the problem, let be be …
Bjorn Poonen's user avatar
  • 23.8k
3 votes

Can I detect the point of impact without looking at it?

The answer is that everything can be recovered, if $f$ is chosen suitably! There are $2^{\aleph_0}$ possibilities for $c$, so we can fix an injection $c \mapsto u_c$ from the set of possible $c$ into …
Bjorn Poonen's user avatar
  • 23.8k