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M. Rudnev

Publications and source records attributed to M. Rudnev.

8 recordsLinked to original sources
arXiv2013

Group actions and geometric combinatorics in ${\mathbb F}_q^d$

In this paper we apply a group action approach to the study of Erd\H os-Falconer type problems in vector spaces over finite fields and use it to obtain non-trivial exponents for the distribution of simplices. We prove that there exists $s_0(d)<d$ such that if $E \subset {\mathbb F}_q^d$, $d \ge 2$, with $|E| \ge Cq^{s_0}$, then $|T^d_d(E)| \ge C'q^{d+1 \choose 2}$, where $T^d_k(E)$ denotes the set of congruence classes of $k$-dimensional simplices determined by $k+1$-tuples of points from $E$. Non-trivial exponents were previously obtained by Chapman, Erdogan, Hart, Iosevich and Koh (\cite{CEHIK12}) for $T^d_k(E)$ with $2 \leq k \leq d-1$. A non-trivial result for $T^2_2(E)$ in the plane was obtained by Bennett, Iosevich and Pakianathan (\cite{BIP12}). These results are significantly generalized and improved in this paper. In particular, we establish the Wolff exponent $\frac{4}{3}$, previously established in \cite{CEHIK12} for the $q\equiv3\mbox{ mod }4$ case to the case $q\equiv1\mbox{ mod }4$, and this results in a new sum-product type inequality. We also obtain non-trivial results for subsets of the sphere in ${\mathbb F}_q^d$, where previous methods have yielded nothing. The key to our approach is a group action perspective which quickly leads to natural and effective formulae in the style of the classical Mattila integral from geometric measure theory.

math.CO↗
arXiv2005

On inverse problem of dynamics

We study the question whether for a natural Hamiltonian system on a two-dimensional compact configuration manifold, a single trajectory of sufficiently high energy is almost surely enough to reconstruct a real analytic potential.

math.DS↗
arXiv2005

A model for separatrix splitting near multiple resonances

We propose a model for local dynamics of a perturbed convex real-analytic Liouville-integrable Hamiltonian system near a resonance of multiplicity $1+m, m\geq 0$. Physically, the model represents a toroidal pendulum, coupled with a Liouville-integrable system of $n$ non-linear rotators via a small analytic potential. The global bifurcation problem is set-up for the $n$-dimensional isotropic manifold, corresponding to a specific homoclinic orbit of the toroidal pendulum. The splitting of this manifold can be described by a scalar function on an $n$-torus, whose $k$th Fourier coefficient satisfies the estimate $$O(e^{- ρ|k\cdotω| - |k|σ}), k\in\Z^n\setminus\{0\},$$ where $ω\in\R^n$ is a Diophantine rotation vector of the system of rotators; $ρ\in(0,{π\over2})$ and $σ>0$ are the analyticity parameters built into the model. The estimate, under suitable assumptions would generalize to a general multiple resonance normal form of a convex analytic Liouville integrable Hamiltonian system, perturbed by $O(\eps)$, in which case $ω_j\sim\omeps, j=1,...,n.$

math.DS↗
arXiv2004

A new integrable 3+1 dimensional generalization of the Burgers equation

A new nonlinear 3+1 dimensional evolution equation admitting the Lax pair is presented. In the case of one spatial dimension, the equation reduces to the Burgers equation. A method of construction of exact solutions, based on a class of discrete symmetries of the former equation is developed. These symmetries reduce to the Cole-Hopf transformation in one-dimensional limit. Some exact solutions are analyzed, in the physical context of spatial dissipative structures and shock wave dressing.

nlin.SI↗
arXiv2004

Endpoint bounds for the non-isotropic Falconer distance problem associated with lattice-like sets

Let $S \subset {\mathbb R}^d$ be contained in the unit ball. Let $Δ(S)=\{||a-b||:a,b \in S\}$, the Euclidean distance set of $S$. Falconer conjectured that the $Δ(S)$ has positive Lebesque measure if the Hausdorff dimension of $S$ is greater than $\frac{d}{2}$. He also produced an example, based on the integer lattice, showing that the exponent $\frac{d}{2}$ cannot be improved. In this paper we prove the Falconer distance conjecture for this class of sets based on the integer lattice. In dimensions four and higher we attain the endpoint by proving that the Lebesgue measure of the resulting distance set is still positive if the Hausdorff dimension of $S$ equals $\frac{d}{2}$. In three dimensions we are off by a logarithm. More generally, we consider $K$-distance sets $Δ_K(S)=\{{|a-b|}_K: a,b \in S\}$, where ${|\cdot|}_K$ is the distance induced by a norm defined by a smooth symmetric convex body $K$ whose boundary has everywhere non-vanishing Gaussian curvature. We prove that our endpoint result still holds in this setting, providing a further illustration of the role of curvature in this class of problems.

math.CA↗
arXiv2002

Darboux transformation for classical acoustic spectral problem

We study discrete isospectral symmetries for the classical acoustic spectral problem in spatial dimensions one and two, by developing a Darboux (Moutard) transformation formalism for this problem. The procedure follows the steps, similar to those for the Schrödinger operator. However, there is no one-to-one correspondence between the two problems. The technique developed enables one to construct new families of integrable potentials for the acoustic problem, in addition to those already known. The acoustic problem produces a non-linear Harry Dym PDE. Using the technique, we reproduce a pair of simple soliton solutions of this equation. These solutions are further used to construct a new positon solution for this PDE. Furthermore, using the dressing chain approach, we build a modified Harry Dym equation together with its LA-pair. As an application, we construct some singular and non-singular integrable potentials (dielectric permitivity) for the Maxwell equations in a 2D inhomogeneous medium.

nlin.SI↗