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Rajula Srivastava

Publications and source records attributed to Rajula Srivastava.

15 recordsLinked to original sources
arXiv2026

Sharp Bounds for Rational Points Near Space Curves

Let $Q\geq 1$ be large, and $δ\in(0,1)$ be small. Denote by $\mathcal C \subset \mathbb R^3$ a sufficiently smooth curve with non-vanishing curvature and torsion. How many rational points $\mathbf{a}/q$ of height $q\in[1, Q]$ are $δ/q$-near $\mathcal C$? This manuscript provides an essentially optimal answer, thereby addressing a problem stated by Beresnevich and Kleinbock, for space curves. We show that the folklore conjectures are incorrect for certain manifolds with codimension $\ge 2$, including the moment curve $(t,t^2,t^3)$. The reason is a {hitherto} hidden `major arc' type obstruction. We also establish matching upper bounds, up to endpoints. Our argument combines purely Fourier analytical techniques with the planar counting results by Vaughan and Velani.

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arXiv2026

Rational Points in Hyperbolic Regions and Multiplicative Diophantine Approximation on Manifolds

We establish the convergence theory of multiplicative Diophantine approximation for all non-degenerate, smooth manifolds. We also settle said convergence theory for all affine subspaces satisfying a highly generic and essentially optimal Diophantine condition. This answers a question of Beresnevich and Velani from 2005, while simultaneously sharpening results of Kleinbock and Margulis on the strong extremality of non-degenerate manifolds, and of Kleinbock on the strong extremality of affine subspaces.

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arXiv2025

Counting rational points near manifolds: a refined estimate, a conjecture and a variant

Refining an argument of the second author, we improve the known bounds for the number of rational points near a submanifold of $\mathbb{R}^d$ of intermediate dimension under a natural curvature condition. Furthermore, in the codimension $2$ case we formulate a conjecture concerning this count. The conjecture is motivated in part by interpreting certain codimension $2$ submanifolds of $\mathbb{R}^{2m+2}$ as complex hypersurfaces in $\mathbb{C}^{m+1}$ and using the complex structure to provide a natural reformulation of the curvature condition. Finally, we provide further evidence for the conjecture by proving a natural variant for $n \geq 2$ in which rationals are replaced with Gaussian rationals.

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arXiv2025

Carleson operators on doubling metric measure spaces

Doubling metric measure spaces provide a natural framework for singular integral operators. In contrast, the study of maximally modulated singular integral operators, the so-called Carleson operators, has largely been limited to Euclidean space with modulation functions such as polynomials defined by algebraic means. We present a general axiomatic approach to modulation functions on doubling metric measure spaces and prove $L^p$ bounds for the corresponding Carleson operators in Theorem 1.1 and Theorem 1.2. This generalizes classical and modern results on Carleson operators. In addition to the proofs presented here, our main results have been computer verified using the language Lean and the library mathlib, as documented in the sibling communication arXiv:2405.06423.

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arXiv2025

A blueprint for the formalization of Carleson's theorem on convergence of Fourier series

This paper is the blueprint underlying the Lean formalization of the proof of Carleson's classical result asserting almost everywhere convergence of Fourier series of continuous functions. We break up the proof into two steps, a reduction of the classical result to a new theorem that appears in a sibling communication and a proof of this new theorem, which is also detailed as blueprint in this paper. An early version of this blueprint was used to initiate the Lean formalization. During the formalization, many contributors elaborated the blueprint with minor corrections, modifications and extensions. The final version is presented here as a guide through the accompanying Lean code.

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arXiv2025

Counting integral points near space curves: a Fourier analytic approach

We establish upper and lower bounds for the number of integral points which lie within a neighbourhood of a smooth nondegenerate curve in $\mathbb{R}^n$ for $n\geq 3$. These estimates are new for $n\geq 4$, and we recover an earlier result of J. J. Huang for $n=3$. However, we do so by using Fourier analytic techniques which, in contrast with the method of Huang, do not require the sharp counting result for planar curves as an input. In particular, we rely on an Arkhipov--Chubarikov--Karatsuba-type oscillatory integral estimate.

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arXiv2025

Counting Rational Points In Non-Isotropic Neighborhoods of Manifolds

In this manuscript, we initiate the study of the number of rational points with bounded denominators, contained in a non-isotropic $δ_1\times\ldots\times δ_R$ neighborhood of a compact submanifold $\mathcal{M}$ of codimension $R$ in $\mathbb{R}^{M}$. We establish an upper bound for this counting function which holds when $\mathcal{M}$ satisfies a strong curvature condition, first introduced by Schindler-Yamagishi in \cite{schindler2022density}. Further, even in the isotropic case when $δ_1=\ldots=δ_R=δ$, we obtain an asymptotic formula which holds beyond the range of distance to $\mathcal{M}$ established in \cite{schindler2022density}. Our result is also a generalization of the work of J.J. Huang \cite{huangduke} for hypersurfaces. As an application, we establish for the first time an upper bound for the Hausdorff dimension of the set of weighted simultaneously well approximable points on a manifold $\mathcal{M}$ satisfying the strong curvature condition, which agrees with the lower bound obtained by Allen-Wang in \cite{allen2022note}. Moreover, for $R>1$, we obtain a new upper bound for the number of rational points \textit{on} $\mathcal{M}$, which goes beyond the bound in an analogue of Serre's dimension growth conjecture for submanifolds of $\mathbb{R}^M$ .

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arXiv2025

Counting Lattice Points near Korányi Spheres via Generalized Radon Transforms

In this note, we study a lattice point counting problem for spheres in Heisenberg groups, incorporating both the non-isotropic dilation structure and the non-commutative group law. More specifically, we establish an upper bound for the average number of lattice points in a $δ$-neighborhood of a Korányi sphere of large radius, where the average considered is over Heisenberg group translations of the sphere. This is in contrast with previous works, which either count lattice points on dilates of a fixed sphere (see \cites{GNT, Gath2}) or consider averages over Euclidean translations of the sphere (see \cites{CT}). We observe that incorporating the Heisenberg group structure allows us to circumvent the degeneracy arising from the vanishing of the Gaussian curvature at the poles of the Korányi sphere. In fact, in lower dimensions (the first and second Heisenberg group), our method establishes an upper bound for this number which gives a logarithmic improvement over the bound implied by the previously known results. Even for the higher dimensional Heisenberg groups, we recover the bounds implied by the main result of \cite{GNT} using a completely different approach of generalized Radon transforms. Further, we obtain upper bounds for the average number of lattice points near more general spheres described with respect to radial, Heisenberg homogeneous norms as considered in \cite{GNT}.

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arXiv2024

Density of Rational Points Near Flat/Rough Hypersurfaces

For $n\geq 3$, let $\mathscr{M} \subseteq\mathbb{R}^{n}$ be a compact hypersurface, parametrized by a homogeneous function of degree $d\in \mathbb{R}_{>1}$, with non-vanishing curvature away from the origin. Consider the number $\mathrm{N}_{\mathscr{M}}(δ,Q)$ of rationals $\mathbf{a}/q$, with denominator $q\in [Q,2Q)$ and $\mathbf{a} \in \mathbb{Z}^{n-1}$, lying at a distance at most $δ/q$ from $\mathscr{M}$. This manuscript provides essentially sharp estimates for $\mathrm{N}_{\mathscr{M}}(δ,Q)$ throughout the range $δ\in (Q^{\varepsilon-1},1/2)$ for $d>1+\tfrac{1}{2n-3}$. Our result is a first of its kind for hypersurfaces with vanishing Gaussian curvature ($d>2$) and those which are rough (meaning not even $C^2$ at the origin which happens when $d<2$). An interesting outcome of our investigation is the understanding of a `geometric' term $(δ/Q)^{(n-1)/d}Q^n$ (stemming from a so-called Knapp cap), arising in addition to the usual probabilistic term $δQ^n$; the sum of these terms determines the size of $\mathrm{N}_{\mathscr{M}}(δ,Q)$ for $δ\in(Q^{\varepsilon-1},1/2)$. Consequences of our result concern the metric theory of Diophantine approximation on `rough' hypersurfaces -- going beyond the recent break-through of Beresnevich and L. Yang. Further, we establish smooth extensions of Serre's dimension growth conjecture.

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arXiv2023

Rational Points Near Manifolds, Homogeneous Dynamics, and Oscillatory Integrals

Let $\mathcal{M}\subset \mathbb{R}^n$ be a compact and sufficiently smooth manifold of dimension $d$. Suppose $\mathcal{M}$ is nowhere completely flat. Let $N_{\mathcal{M}}(δ,Q)$ denote the number of rational vectors $\mathbf{a}/q$ within a distance of $δ/q$ from $\mathcal{M}$ so that $q \in [Q,2Q)$. We develop a novel method to analyse $N_{\mathcal{M}}(δ,Q)$. The salient feature of our technique is the combination of powerful quantitative non-divergence estimates, in a form due to Bernik, Kleinbock, and Margulis, with Fourier analytic tools. The second ingredient enables us to eschew the Dani correspondence and an explicit use of the geometry of numbers. We employ this new method to address in a strong sense a problem of Beresnevich regarding lower bounds on $N_{\mathcal{M}}(δ,Q)$ for non-analytic manifolds. Additionally, we obtain asymptotic formulae which are the first of their kind for such a general class of manifolds. As a by-product, we improve upon upper bounds on $N_{\mathcal{M}}(δ,Q)$ from a recent breakthrough of Beresnevich and Yang and recover their convergence Khintchine type theorem for arbitrary nondegenerate submanifolds. Moreover, we obtain new Hausdorff dimension and measure refinements for the set of well-approximable points for a range of Diophantine exponents close to $1/n$.

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arXiv2023

Spherical maximal operators on Heisenberg groups: Restricted dilation sets

Consider spherical means on the Heisenberg group with a codimension two incidence relation, and associated spherical local maximal functions $M_Ef$ where the dilations are restricted to a set $E$. We prove $L^p\to L^q$ estimates for these maximal operators; the results depend on various notions of dimension of $E$.

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arXiv2022

On the Korányi Spherical maximal function on Heisenberg groups

We prove $L^p\to L^q$ estimates for the local maximal operator associated with dilates of the Kóranyi sphere in Heisenberg groups. These estimates are sharp up to endpoints and imply new bounds on sparse domination for the corresponding global maximal operator. We also prove sharp $L^p\to L^q$ estimates for spherical means over the Korányi sphere, which can be used to improve the sparse domination bounds for the associated lacunary maximal operator.

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arXiv2021

Lebesgue space estimates for spherical maximal functions on Heisenberg groups

We prove $L^p\to L^q$ estimates for local maximal operators associated with dilates of codimension two spheres in Heisenberg groups; these are sharp up to two endpoints. The results can be applied to improve currently known bounds on sparse domination for global maximal operators. We also consider lacunary variants, and extensions to Métivier groups.

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arXiv2020

Orthogonal Systems of Spline Wavelets as Unconditional Bases in Sobolev Spaces

We exhibit the necessary range for which functions in the Sobolev spaces $L^s_p$ can be represented as an unconditional sum of orthonormal spline wavelet systems, such as the Battle-Lemarié wavelets. We also consider the natural extensions to Triebel-Lizorkin spaces. This builds upon, and is a generalization of, previous work of Seeger and Ullrich, where analogous results were established for the Haar wavelet system.

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