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

10 votes
2 answers
341 views

Is this Riccati equation ("Josephson junction") always phase-locked at integer rotation numb...

Given parameters $(a,k,A) \in \mathbb{R}^3$, we consider on $\mathbb{S}^1$ the $2\pi$-periodic ODE $$ \dot{\theta} \ = \ - a\sin(\theta) + k + A\cos(t) \hspace{4mm} \mathrm{mod} \ 2\pi. $$ Identifying …
Julian Newman's user avatar
1 vote
1 answer
122 views

Can a (non-measurable) autonomous flow have a non-trivial periodic orbit without a minimal p...

Given a set $X$, a function $x \colon \mathbb{R} \to X$ is periodic if there exists $\tau>0$ such that $x(t+\tau)=x(t)$ for all $t \in \mathbb{R}$; and if $\tau$ is the smallest positive number with t …
Julian Newman's user avatar
2 votes
0 answers
228 views

Functions with "gradients of bounded variation"

Dear all, I would like to know whether the following concept is one that is commonly studied, or has a name, or if there are any textbooks that make reference to it: We say that a function $f:[a,b] …
Julian Newman's user avatar
1 vote
1 answer
193 views

Can you control the amplitudes of a finite collection of sine curves just by controlling the...

Dear all, I would like to know whether the following claim is true. In particular, if it is true, then I would like to know if there is some textbook that contains the statement and maybe even the pr …
Julian Newman's user avatar