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1. Diffusions interacting through a random matrix: universality via stochastic Taylor expansion

   https://doi.org/10.1007/s00440-021-01027-7  

   Dembo, Amir; Gheissari, Reza (August 2021, Probability Theory and Related Fields)

   Abstract Consider $$(X_{i}(t))$$ ( X i ( t ) ) solving a system of N stochastic differential equations interacting through a random matrix $${\mathbf {J}} = (J_{ij})$$ J = ( J ij ) with independent (not necessarily identically distributed) random coefficients. We show that the trajectories of averaged observables of $$(X_i(t))$$ ( X i ( t ) ) , initialized from some $$\mu $$ μ independent of  $${\mathbf {J}}$$ J , are universal, i.e., only depend on the choice of the distribution $$\mathbf {J}$$ J through its first and second moments (assuming e.g., sub-exponential tails). We take a general combinatorial approach to proving universality for dynamical systems with random coefficients, combining a stochastic Taylor expansion with a moment matching-type argument. Concrete settings for which our results imply universality include aging in the spherical SK spin glass, and Langevin dynamics and gradient flows for symmetric and asymmetric Hopfield networks. 

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2. Fast Regression for Structured Inputs

   Meyer, Raphael; Musco, Cameron; Musco, Christopher; Woodruff, David P.; Zhou, Samson (April 2022, International Conference on Learning Representations (ICLR))

   We study the $$\ell_p$$ regression problem, which requires finding $$\mathbf{x}\in\mathbb R^{d}$$ that minimizes $$\|\mathbf{A}\mathbf{x}-\mathbf{b}\|_p$$ for a matrix $$\mathbf{A}\in\mathbb R^{n \times d}$$ and response vector $$\mathbf{b}\in\mathbb R^{n}$$. There has been recent interest in developing subsampling methods for this problem that can outperform standard techniques when $$n$$ is very large. However, all known subsampling approaches have run time that depends exponentially on $$p$$, typically, $$d^{\mathcal{O}(p)}$$, which can be prohibitively expensive. We improve on this work by showing that for a large class of common \emph{structured matrices}, such as combinations of low-rank matrices, sparse matrices, and Vandermonde matrices, there are subsampling based methods for $$\ell_p$$ regression that depend polynomially on $$p$$. For example, we give an algorithm for $$\ell_p$$ regression on Vandermonde matrices that runs in time $$\mathcal{O}(n\log^3 n+(dp^2)^{0.5+\omega}\cdot\text{polylog}\,n)$$, where $$\omega$$ is the exponent of matrix multiplication. The polynomial dependence on $$p$$ crucially allows our algorithms to extend naturally to efficient algorithms for $$\ell_\infty$$ regression, via approximation of $$\ell_\infty$$ by $$\ell_{\mathcal{O}(\log n)}$$. Of practical interest, we also develop a new subsampling algorithm for $$\ell_p$$ regression for arbitrary matrices, which is simpler than previous approaches for $$p \ge 4$$. 

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3. Stability of a point charge for the repulsive Vlasov–Poisson system

   https://doi.org/10.4171/JEMS/1518  

   Pausader, Benoît; Widmayer, Klaus; Yang, Jiaqi (August 2024, Journal of the European Mathematical Society)

   We consider solutions of the repulsive Vlasov–Poisson system which are a combination of a point charge and a small gas, i.e., measures of the form\delta_{(\mathcal{X}(t),\mathcal{V}(t))}+\mu^{2}d\mathbf{x}d\mathbf{v}for some(\mathcal{X}, \mathcal{V})\colon \mathbb{R}\to\mathbb{R}^{6}and a small gas distribution\mu\colon \mathbb{R}\to L^{2}_{\mathbf{x},\mathbf{v}}, and study asymptotic dynamics in the associated initial value problem. If initially suitable moments on\mu_{0}=\mu(t=0)are small, we obtain a global solution of the above form, and the electric field generated by the gas distribution \mudecays at an almost optimal rate. Assuming in addition boundedness of suitable derivatives of \mu_{0}, the electric field decays at an optimal rate, and we derive modified scattering dynamics for the motion of the point charge and the gas distribution. Our proof makes crucial use of the Hamiltonian structure. The linearized system is transport by the Kepler ODE, which we integrate exactly through an asymptotic action-angle transformation. Thanks to a precise understanding of the associated kinematics, moment and derivative control is achieved via a bootstrap analysis that relies on the decay of the electric field associated to\mu. The asymptotic behavior can then be deduced from the properties of Poisson brackets in asymptotic action coordinates. 

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4. Subconvexity in Inhomogeneous Vinogradov Systems

   https://doi.org/10.1093/qmath/haac027  

   Wooley, Trevor D. (August 2022, The Quarterly Journal of Mathematics)

   Abstract When k and s are natural numbers and $${\mathbf h}\in {\mathbb Z}^k$$, denote by $$J_{s,k}(X;\,{\mathbf h})$$ the number of integral solutions of the system $$ \sum_{i=1}^s(x_i^j-y_i^j)=h_j\quad (1\leqslant j\leqslant k), $$ with $$1\leqslant x_i,y_i\leqslant X$$. When $$s\lt k(k+1)/2$$ and $$(h_1,\ldots ,h_{k-1})\ne {\mathbf 0}$$, Brandes and Hughes have shown that $$J_{s,k}(X;\,{\mathbf h})=o(X^s)$$. In this paper we improve on quantitative aspects of this result, and, subject to an extension of the main conjecture in Vinogradov’s mean value theorem, we obtain an asymptotic formula for $$J_{s,k}(X;\,{\mathbf h})$$ in the critical case $s=k(k+1)/2$. The latter requires minor arc estimates going beyond square-root cancellation. 

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5. Holomorphic curves in the 6-pseudosphere and cyclic surfaces

   https://doi.org/10.1090/tran/9172  

   Collier, Brian; Toulisse, Jérémy (June 2024, Transactions of the American Mathematical Society)

   The space ${\mathbf{H}}^{4,2}$of vectors of norm $-<\#comment/>1$in ${\mathbb{R}}^{4,3}$has a natural pseudo-Riemannian metric and a compatible almost complex structure. The group of automorphisms of both of these structures is the split real form ${\mathsf{G}}_2^{\prime }$. In this paper we consider a class of holomorphic curves in ${\mathbf{H}}^{4,2}$which we call alternating. We show that such curves admit a so called Frenet framing. Using this framing, we show that the space of alternating holomorphic curves which are equivariant with respect to a surface group is naturally parameterized by certain ${\mathsf{G}}_2^{\prime }$-Higgs bundles. This leads to a holomorphic description of the moduli space as a fibration over Teichmüller space with a holomorphic action of the mapping class group. Using a generalization of Labourie’s cyclic surfaces, we then show that equivariant alternating holomorphic curves are infinitesimally rigid. 

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