Peer Revenue Publish or profit

Papers

Fresh from arXiv, typeset properly. Highlighted words trade on the market.

Mathematics Classical Analysis and ODEs

math.CA Watch Classical Analysis and ODEs (sign in to use)

Wednesday 12 August

Classical Analysis and ODEs

Lommel polynomials and explicitly solvable prediction problems on the unit circle

To each finite symmetric measure $\sigma$ on the real line, with support a compact subset of $(-2,2)$, we associate a measure $\mu$ on the unit circle by transporting mass at $\pm x$ to $e^{\pm i\theta(x)}$, $\theta(x)=2\arcsin(x/2)$. The linear prediction errors, Verblunsky coefficients, and Toeplitz determinants of $\mu$ are then expressed through the orthogonal polynomial data of $\sigma$ at the single edge point $x=2$. In particular $E_m(\mu)=\tfrac12 t_m\|P_m\|_\sigma^2$ with $t_m=P_{m+1}(2)/P_m(2)$. Under a condition on the first coefficients, decay of the recurrence coefficients of $\sigma$ forces the $t_m$ to increase from $t_1$ onward, a Turán-type monotonicity in the degree placing every prediction margin of $\mu$ past the first above the corresponding coefficient of $\sigma$. Taking $\sigma$ to be the Lommel-polynomial measures, with atoms at rescaled reciprocals of the zeros of the Bessel function $J_\nu$, yields a two-parameter family of purely atomic circle measures with a normalized determinant limit equal to a value of $J_\nu$.
Read the full abstract

Classical Analysis and ODEs

Near optimal three-fold additive energy bound for points on convex curves

Let $X\subset\mathbb{R}$ be finite and let $\gamma(t)=(t,f(t))$, where $f$ is strictly convex. We show that \[ J_3(\gamma(X)) =\#\{(x_1,\ldots,x_6)\in X^6:\sum_{i=1}^3\gamma(x_i)=\sum_{i=4}^6\gamma(x_i)\} \ll_{\epsilon}|X|^{3+\epsilon}. \] When specialized to the parabola, our result implies near-optimal estimates for the number of solutions to the diameter-free quadratic Vinogradov system. As a second application, we settle a conjecture from Krishnapur-Kurlberg-Wigman and Bombieri-Bourgain concerning lattice points on dilates of the unit circle. As a third application, we prove that $|A-A|\gg_\epsilon|A|^{5/3-\epsilon}$ and $|A+A|\gg_\epsilon|A|^{8/5-\epsilon}$ for any finite convex sequence $A\subset \mathbb{R}$.
Read the full abstract

Classical Analysis and ODEs

On the optimal constant in the Montgomery-Vaughan weighted Hilbert inequality

We give a proof that the optimal constant in the weighted Hilbert inequality is strictly greater than $\pi$, answering in the negative a question asked by Montgomery and Vaughan. The proof proceeds via an explicit limiting counterexample.

Classical Analysis and ODEs · Cross-listed from math.FA

Grothendieck's theorem for Bessel sequences

We establish a sharp version of Grothendieck's theorem for Bessel sequences. Precisely, given a Bessel sequence $\{ x_j \}_{j\in\mathbb{N}}$ with Bessel bound $1$ in a Hilbert space, we show that there exists functions $\{ f_j \}_{j\in\mathbb{N}}$ belonging to the unit ball of $L^\infty([0,1])$ such that for all $j,k \in \mathbb{N}$ one has $$ \langle x_j,x_k\rangle = \int_0^1 f_j(x)\overline{f_k(x)}\,dx.$$ As an application, we give an affirmative answer to an extension problem of Olevskii: if $E \subset [0,1]$ is a Lebesgue measurable set such that $[0,1]\setminus E$ has positive measure, then every Bessel sequence in $L^2(E)$ with Bessel bound $1$ extends to an orthonormal system in $L^2([0,1])$ that is bounded by the (optimal) constant $\lambda([0,1]\setminus E)^{-1/2}$ on $[0,1]\setminus E$. A formalization of our main result in Lean 4 accompanies the paper.
Read the full abstract

Classical Analysis and ODEs · Cross-listed from math.CV

The Corona Problem for Slice Holomorphic Functions via the Matrix Corona Problem

In this paper we prove the Corona theorem for slice hyperholomorphic functions in a general way that applies to functions with values in a Clifford algebra $\mathbb R_n$ for any $n\geq 2$. This general proof is based on a matrix version of Corona's theorem and then exploiting some symmetries of the matrices that allow to reconstruct the values of the functions. The crucial idea is to tie together the algebra of the functions, the Bezout identities and the Carleson-type conditions. The results we obtain rest on a new way of writing the slice product which improves the standard approach and which holds in general dimension.
Read the full abstract

All authors Fabrizio Colombo, Elodie Pozzi, Irene Sabadini, Brett D. Wick

Classical Analysis and ODEs · Cross-listed from quant-ph

Isospectral majorization and isoperimetric inequalities for coherent states on the Bloch sphere

Let $\mathcal{P}_{N}$ be the $(N+1)$-dimensional Hilbert space of analytic polynomials of degree at most $N$. This is the natural environment to define $SU(2)$ (Bloch) coherent states. Let $Q_{\rho }$ be the Husimi function of a density operator $\rho $ on $% \mathcal{P}_{N}$. We prove an isospectral version of Lieb-Solovej inequality: if $\rho ^{\downarrow }$ is obtained by placing the eigenvalues of $\rho $ in decreasing order along the monomial basis, then \begin{equation*} \int_{\mathbb{C}}\Phi (Q_{\rho }(z))\,dm(z)\leq \int_{\mathbb{C}}\Phi (Q_{\rho ^{\downarrow }}(z))\,dm(z) \end{equation*}% for every convex function $\Phi $ on $[0,1]$. Applying the corresponding reversed inequality to the concave function $\Phi (t)=-t\log t$ gives the Wehrl entropy. In the process it is show that the output state of $\rho $ under Lieb-Solovej's channel is majorized by the output of the state $\rho ^{\downarrow }$. As an application, among all measurable subsets of the sphere having a fixed area, spherical caps maximize every partial sum of the eigenvalues of Toeplitz operators with symbol $1_{\Omega}$. Equivalently, caps maximize all Ky Fan norms, leading to the sharp inequalities, previously known for $r=1$: \begin{equation*} \sum_{j=1}^{r}\lambda_{j}(\Omega) \leq r-\sum_{k=0}^{r-1}(r-k)\binom{N+1}{k} m(\Omega)^{k}\bigl(1-m(\Omega)\bigr)^{N+1-k}. \end{equation*} This implies isoperimetric inequalities for all Schatten sums of $T_{\Omega}$, obtained without using the spherical isoperimetric inequality.
Read the full abstract

Classical Analysis and ODEs · Cross-listed from math.NT

The 196560 auxiliary-function conjecture for the Leech lattice

Cohn and Kumar conjectured in 2009 that there is a radial Schwartz function $g\colon\R^{24}\to\R$ satisfying $g(r)\leq0$ for $r\geq\sqrt6$, $\widehat g(r)\geq0$ for $r\geq0$, $g(2)>0$, and $(\widehat g(0)-g(0))/g(2)=196560$. We construct such functions from the radial Fourier interpolation basis in dimension $24$. If $a_2,b_2$ denote the basis functions dual to value and radial-derivative interpolation at radius $2$, then $g_C=a_2-Cb_2$ has exactly the nodal data needed for Poisson summation over the Leech lattice. The sphere-packing magic function identifies $b_2$ and supplies its strict signs. We prove that the removable quotients $a_2/b_2$ and $\widehat a_2/\widehat b_2$ are bounded on the required half-lines. The noncompact step follows from exact coefficient extraction in the interpolation kernel and an $S$-cusp expansion. Both quotients tend to $(43+240\log2)/15$; for all sufficiently large radii they lie on opposite sides of this limit, with an explicit first exponential correction. Consequently the admissible parameters in this affine family form a nonempty closed ray, and every member proves the conjectured identity. The same interpolation basis recovers every nontrivial Leech-shell coefficient.
Read the full abstract

Classical Analysis and ODEs

On Constrained Riesz Minimum Energy Problems

The constrained equilibrium problem for the logarithmic potential was introduced by Rakhmanov (1996) as he realized that the asymptotic distribution of the zeros of discrete orthogonal polynomials in a compact interval could be described in terms of the equilibrium measure of this interval in a class of measures subject to a certain constraint. Other authors such as Dragnev and Saff (1997), and Kuijlaars and Van Assche (1999), extended this approach to more general settings. These problems have proven to be useful for describing asymptotic distributions in different settings. In the current paper, we consider constrained equilibrium problems in the Riesz setting, that is, for $s$-Riesz potentials in the hyperplane $\mathbb{R}^d,$ with $d\geq 1$ and $\max (0,d-2) < s < d$. Along with some general results, an illustrative example consisting of the solution of a constrained equilibrium problem in the unit ball is studied in detail. This is the main part of the paper. Finally, a number of numerical experiments show how the constrained equilibrium measure, the solution of the problem, may be discretized using the so-called 'constrained Leja points' introduced by Coroian and Dragnev (2001).
Read the full abstract

All authors Dan Coroian, Peter Dragnev, Alan Legg, Ramon Orive

Classical Analysis and ODEs

Kakeya sets and dimension compression in compact Lie groups

We study two Kakeya set problems in compact semisimple Lie groups. In the first, motivated by the group structure, a Kakeya set is required to contain a left coset of every closed one-dimensional torus. For a compact semisimple Lie group $G$ of dimension $d$ and rank $r$, we determine the optimal Minkowski dimension for this problem, proving that it is exactly $(d+r)/2$. The upper bound is obtained from a Lie-theoretic construction associated with a Chevalley involution, while the lower bound combines incidence geometry with geometry of numbers. We also consider a local Kakeya set problem, closer to the classical harmonic-analytic formulation, in which one requires a fixed-length one-parameter arc in every direction. For this problem we obtain lower bound $(d+1)/2$ and $(d+2)/2$ for odd and even $r$, and upper bound $(d+r)/2$. We conjecture that the upper bound should be sharp.
Read the full abstract

Tuesday 11 August

Classical Analysis and ODEs · Cross-listed from math.DS

A Green Function Approach to Smooth Nonautonomous Topological Equivalence with Unbounded Nonlinearities under $(\mu,\nu)$–Dichotomies

We study the smooth topological equivalence between a nonautonomous linear system on the positive half-line and a quasilinear perturbation whose nonlinear part is not assumed to be globally bounded with respect to the state variable. The linear equation is assumed to admit a $(\mu,\nu)$–dichotomy, and the construction is carried out through the Green operator associated with this dichotomy. The usual global boundedness of the perturbation is replaced by locally uniform Green-integrability conditions along the relevant linear and nonlinear solutions. Under a smallness condition involving the global Lipschitz constant of the perturbation and the Green operator, we first construct Palmer-type maps which give a continuous topological equivalence on $\mathbb R^+$. We then impose Green-integrability conditions on the successive variational equations and a further first-order smallness condition which guarantees the invertibility of the derivative of the inverse map. Under these assumptions, the equivalence is of class $C^r$. We also provide a class of examples for which the perturbation is unbounded with respect to the space variable and all the hypotheses can be verified directly.
Read the full abstract

Classical Analysis and ODEs · Cross-listed from math.DS

On the Assouad dimension of Weierstrass function graphs

The class of Weierstrass functions $W_{a,b}$ is one of the first class of examples of continuous and nowhere differentiable real functions. A challenging line of research has been to determine the various dimensions of graphs $G(W_{a,b})$ of such functions. For instance, the Hausdorff dimension of $G(W_{a,b})$ was only recently determined by Shen in 2018, after a long series of partial results by many different authors. While the Assouad dimension of $G(W_{a,b})$ remains an open problem, also posed as a question by J. M. Fraser, there have been many indications that it might be equal to $2$. Such indications include the graph of Wiener processes, the graphs of almost all Hölder functions in the Baire category sense, and the graphs of Weierstrass functions after a series of countably many reflections all having Assouad dimension equal to $2$. In this paper we show that this is not the case, providing a quantitative upper bound on the Assouad dimension of $G(W_{a,b})$ that is strictly less than $2$. In particular, we show that such a bound is true for a class of generalized Weierstrass functions $W_{a,b}^\phi(x) = \sum_{j=0}^\infty a^j\phi(b^j x)$, which includes $W_{a,b}$ and the class of Takagi functions. The latter fact is used to also answer in the negative a conjecture of H. Yu on the Assouad dimension of graphs of Takagi functions for parameters $a\in (0,1)$, $b\in (1/a, \infty)\cap \mathbb{Z}$.
Read the full abstract

Classical Analysis and ODEs

Matrix Szegő Function and Matrix Orthogonal Polynomials for Multiple Cuts

Following recent developments on matrix-valued orthogonal polynomials (MVOPs), we construct the matrix Szegő factorisation of the matrix weight function when it is supported on multiple intervals. We find that on the gaps the matrix function has unitary multiplicative jumps. In the next part, performing the Deift-Zhou steepest descent analysis we study the associated global parametrix which comes from the Riemann-Hilbert problem of the MVOPs. A solution is constructed visualising the rows of the parametrix as sections of vector bundles on a hyper-elliptic Riemann surface. An alternate view point to the global parametrix is also presented using a vanishing lemma. As an application we obtain strong asymptotics of MVOPs for multiple cuts which extends the works of Deaño, Kuijlaars, and Román.
Read the full abstract

Classical Analysis and ODEs · Cross-listed from math.NT

A double-sum analogue of Whipple's summation formula and a restricted sum formula for multiple binomial sums

We prove a double-sum analogue of Whipple's ${}_3F_2$ summation formula. As an application, we establish a restricted sum formula for multiple binomial sums, which arise as special cases of Igarashi's parametrized multiple zeta series.

Classical Analysis and ODEs

Sharp constants in the one-sided John-Nirenberg inequality for functions of bounded lower oscillation

We determine the sharp constants in the one-sided John-Nirenberg inequality for functions of bounded lower oscillation on an interval. More precisely, if $f\in\BLO(I_0)$, then $$ \sup_{I\subseteq I_0}\frac{1}{|I|} \left|\left\{x\in I: f(x)-\essinf_I f>\lambda\right\}\right| \le c_1\exp\left(-\frac{c_2\lambda}{\norm{f}_{\BLO(I_0)}}\right), \qquad \lambda>0, $$ with the usual interpretation when $\|f\|_{\mathrm{BLO}(I_0)}=0$. The sharp constants are $c_1^*=e$ and $c_2^*=1$. We give a proof based on the Riesz rising sun lemma and an independent Bellman function proof. Finally, we present several consequences of the sharp estimate.
Read the full abstract

Classical Analysis and ODEs

Remarks on a proof of the absolute continuity of the representing measures for Dunkl's intertwining operator

In the published paper [T10], K. Trimèche presented a proof of the statement that for suitable nonnegative multiplicities and regular arguments, the representing measures of Dunkl's intertwining operator and its dual are absolutely continuous with respect to Lebesgue measure. In this note, we argue that the essential proofs of this paper are not correct.
Read the full abstract

Classical Analysis and ODEs

Finite Good Witnesses for Generalized Curve Projections at the Rectifiable Endpoint

For a $1$-rectifiable set $E \subset \mathbb{R}^d$ of positive length, a theorem of Federer shows that, among any $d$ linearly independent orthogonal projections of $E$, at least one has positive length. We develop a version of this finite-witness principle for generalized curve projections. Given scalar-valued mappings $\varphi_1,\ldots,\varphi_d:\mathbb{R}^d\longrightarrow\mathbb{R}$, we introduce the canonical encoding map $\mathsf{H}:=(\varphi_1,\ldots,\varphi_d)$. Where $D\mathsf{H}$ is invertible, a local bilipschitz change of variables and Federer's theorem show that $\varphi_j(E)$ has positive length for some $j$. If a positive-length portion of $E$ lies in the critical set, we instead argue intrinsically on a $C^1$ hypersurface containing it, provided that $\mathsf{H}$ retains full tangential rank there. This yields an abundance of deterministic finite witnesses, as well as structural bounds for those exceptional parameters where the good witness property fails. A new fold non-degeneracy condition makes our constructions stable under perturbations of the underlying parameters. At this rectifiable endpoint, these conclusions complement work of Peres–Schlag, which developed exceptional set estimates for fractal sets of Hausdorff dimension strictly greater than one. We then apply our framework to several nonlinear projection families. Affinely independent pinned squared-distance maps satisfy the tangential and fold conditions, whereas two planar radial projections lose tangential rank along their critical line. We also verify the hypotheses for more exotic examples, including nonlinear anisotropic distances, Bregman functionals arising from smooth approximations of polyhedral norms, and families whose critical hypersurfaces have prescribed $C^2$ geometry.
Read the full abstract

Monday 10 August

Classical Analysis and ODEs · Cross-listed from math.FA

Compact sets in Banach lattices over spaces of homogeneous type

We prove a Kolmogorov–Riesz compactness theorem for Banach lattices over spaces of homogeneous type. Especially, we consider the one for $L^1$. Our characterization is formulated in terms of boundedness, tightness, and approximation by averaging operators, which replace translations in the absence of a group structure. The result requires neither the Vitali covering property nor continuity assumptions on the measure of balls, lower volume bounds, or finiteness of the underlying measure space, thereby extending several recent compactness criteria in the literature. Our approach begins with a new proof of the Kolmogorov–Riesz theorem in $L^1$, which avoids reflexivity and yields a unified treatment of $L^p$-spaces for $1\le p<\infty$. We then establish an approximation theorem based on Christ's dyadic cubes and use it to characterize the closure of compactly supported continuous functions in general ball Banach function spaces. Applications include compactness criteria for rearrangement-invariant Banach function spaces and Morrey spaces. In the Euclidean setting, we also identify the resulting approximation space with the classical heat-semigroup (equivalently, mollifier) closure.
Read the full abstract

All authors Ayşenur Aydoğdu, Amiran Gogatishvili, Yoshihiro Sawano, Daiki Takesako

Classical Analysis and ODEs

Applications of Nonlinear Projections to Rectifiable 1-sets

Projection theorems in Euclidean space provide a fundamental link between the geometric structure of a set and the size of its lower-dimensional images. For 1-rectifiable sets in $\mathbb{R}^d$, a classical theorem of Federer shows that the 1-dimensional Hausdorff measure of such sets is controlled by the multiplicity-weighted lengths of finitely many linearly independent projections. We develop a framework for extending Federer's result into a diverse set of nonlinear problems. This technique yields a unified approach for studying sets through their lower-dimensional nonlinear images, as well as studying the exceptional sets which exhibit poor projective behavior. As illustrations of our technique, we show that (i) every 1-rectifiable set contains a pin whose pinned distance set has positive Lebesgue measure, and that the exceptional set of pins for which this fails is contained in a $(d-2)$-dimensional affine subspace; (ii) planar radial projections of a 1-rectifiable set can fail to have positive length from at most one vantage point unless the set is essentially linear; and finally (iii) unions of circles centered on a 1-rectifiable set have positive area under mild assumptions on the radius function.
Read the full abstract

All authors Rosemarie Bongers, Paige Bright, Caleb Marshall, Krystal Taylor

Classical Analysis and ODEs

Classification of Fourier summation formulas on a horizontal strip

We classify Fourier summation identities in which the measure on the Fourier side is supported in a horizontal strip of $\mathbb{C}$. Let $\mu=\nu+\eta$, where $\nu$ is a strongly tempered measure on $\mathbb{R}$, $\eta=\sum_{m\geq1} b(\gamma_m)\delta_{\gamma_m}$ is a strongly tempered pure point measure supported off the real line, and $a:\Lambda\rightarrow\mathbb{C}$, with $\Lambda=\{\lambda_n\}_{n\geq1}\subset\mathbb{R}$, has finite exponential growth. Under natural real-antipodal and conjugation-symmetry assumptions, we characterize summation identities of the form \begin{align} \sum_{n\geq1} a(\lambda_n)\varphi(\lambda_n)=\int_{\mathbb{R}} \widehat{\varphi}(t)\mathrm{d}\nu(t)+\sum_{m\geq1} b(\gamma_m)\widehat{\varphi}(\gamma_m), \end{align} valid for every $\varphi\in C^\infty_c(\mathbb{R})$, where $\widehat{\varphi}$ denotes the Fourier transform. We prove that each such identity determines a unique generating function $F$ that is holomorphic and almost periodic in the half-plane above the strip and admits a meromorphic continuation to $\mathbb{C}^+$. The measure $\eta$ encodes the poles and residues of $F$, while $\nu$ describes the boundary behavior of its regular part through a generalized Nevanlinna representation, and $a$ determines its Fourier coefficients. Conversely, every function in the corresponding meromorphic class whose Fourier coefficients satisfy a local summability condition determines a unique summation identity of this form. The proof combines a strip version of the Bridge Lemma with a Cauchy-transform argument that accounts for the off-real poles. As an application, we show that the Guinand-Weil explicit formula for every member of the Selberg class, including non-self-dual members, fits into our framework, and we identify its associated generating function.
Read the full abstract

Classical Analysis and ODEs

Almost everywhere convergence of subsequences of Walsh-Nörlund means

Let $\{q_{k}: k\in\mathbb{N}\}$ be a non-increasing and convex sequence of non-negative numbers such that $q_{0}>0$. Our main result is to prove almost everywhere convergence \[ t_{a_{n}}(f)\to f \] of $f\in L_{1}(G)$ as $n\to\infty$, using several $\{a_{n}: n\in\mathbb{P}\}$ subsequences of positive integers for Nörlund means (defined by sequence $q$) on the Walsh-Paley system with weaker conditions than it was known before.
Read the full abstract

Classical Analysis and ODEs

Widom factors for Chebyshev and residual polynomials on semi-regular subsets of $\mathbb{R}$

We study $L^\infty$ Widom factors for Chebyshev and residual polynomials on compact non-polar subsets of the real line that need not be regular in the sense of potential theory and, in particular, on sets with isolated points. We first show that boundedness of the Widom factors is independent of the normalization point $x_*\in\overline{\mathbb{R}}\backslash\mathsf{E}$. For semi-regular sets, meaning that the set of regular points $\mathsf{E}^\mathrm{reg}$ is closed, we introduce the irregularity coefficient $\mathcal{IR}(\mathsf{E},x_*)=\sum_{x\in\mathsf{E}\backslash\mathsf{E}^\mathrm{reg}}G_\mathsf{E}(x,x_*)$, which measures the contribution of irregular boundary points to the extremal polynomial problem. Our upper bound is \[ \sup_{n\ge1}\mathcal{W}_n(\mathsf{E},x_*) \le 2\exp\bigl[\mathcal{PW}(\mathsf{E}^\mathrm{reg},x_*)+\mathcal{IR}(\mathsf{E},x_*)\bigr], \] while the complementary lower bound is \[ \liminf_{n\to\infty}\mathcal{W}_n(\mathsf{E},x_*) \ge 2\exp\bigl[\mathcal{IR}(\mathsf{E},x_*)\bigr]. \] Consequently, for semi-regular Parreau–Widom sets the Widom factors are bounded if and only if $\mathcal{IR}(\mathsf{E},x_*)<\infty$. When the regular part is a finite union of intervals, this condition is equivalent to a geometric square-root summability condition on the irregular points. In the special case $\mathsf{E}=[a,b]\cup\{x_k\}_{k\ge1}$, the Widom factors have the exact, possibly infinite, limit $2\exp[\mathcal{IR}(\mathsf{E},x_*)]$. When this limit is finite, we obtain the corresponding Szegő–Widom asymptotics for the normalized extremal polynomials.
Read the full abstract

Classical Analysis and ODEs

Fourier restriction to hyperbolic rectangles and an application

In this article, we study the Lebesgue space inequalities for extension operators associated with hyperbolic surfaces over rectangular regions. We characterize the corresponding operator norms in terms of the side-lengths. As an application, we present new restriction estimates for a class of hypersurfaces with additive structure.
Read the full abstract

Classical Analysis and ODEs · Cross-listed from math.MG

On the weighted hard-core model and Rado's covering problem for congruent Euclidean balls

Let $K$ be a symmetric convex body in $\mathbb{R}^d$ and let $f(K)$ denote the largest constant $c$ such that every finite collection of translates of $K$ contains a pairwise disjoint subcollection whose total volume is at least $c$ times the volume of the union of the original collection. The classical Vitali covering lemma gives $f(K)\geq3^{-d}$. In this paper, we establish two improvements. First, by a purely combinatorial argument, we prove that $$ f(K)\geq \frac{2}{3^d + 2^d} $$ for every symmetric convex body $K$. This improves the Vitali bound by a factor tending to $2$ as $d$ tends to infinity. Second, using a weighted hard-core model together with a weighted geometric estimate for intersections of Euclidean balls, we show that, for all sufficiently large $d$, $$ f(B^d)\geq \left( \log\frac{3}{1+\sqrt3} -O\left(\frac{\log d}{d}\right) \right)d\,3^{-d}, $$ where $B^d$ is the unit Euclidean ball in $\mathbb{R}^d$. Thus, the classical lower bound is improved by a factor of order $d$.
Read the full abstract

Classical Analysis and ODEs

A new example of crystalline-type measure with unit masses and an application to tiling of the real line by translates of a function

In this note we provide a simple explicit example of a discrete set $\Lambda\subset\mathbb{R}$ of unbounded density such that the Fourier transform of its counting measure $\widehat{\delta}_\Lambda$ is a tempered distribution of the form $c\delta_0'' + \mu$ where $\mu$ is a discrete measure supported by a union of three arithmetic progressions. This example is then used to construct a translational tiling of unbounded density of the real line by a function with compact support.
Read the full abstract

Classical Analysis and ODEs · Cross-listed from math.NT

Sharp Bounds for Rational Points Near Space Curves

Let $Q\geq 1$ be large, and $\delta \in(0,1)$ 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 $\delta/q$-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.
Read the full abstract

All authors Mingfeng Chen, Andreas Seeger, Rajula Srivastava, Niclas Technau

Sunday 9 August

Classical Analysis and ODEs · Cross-listed from math.NT

A conditional arithmetic obstruction to the prime numbers as a spectrum of a probability measure

Let $\Pp=\{2,3,5,7,\ldots\}$ denote the set of prime numbers. We prove that if every sufficiently large positive even integer can be represented as a difference of two primes, then there is no Borel probability measure $\mu$ on $\R$ for which \(\left\{e^{2\pi i p x}:p\in\Pp\right\}\) is an orthonormal basis of $L^2(\mu)$. In particular, under the Polignac conjecture, the prime numbers $\Pp$ cannot be a spectrum (i.e., the set of frequencies of an exponential orthonormal basis) of any probability measure on $\R$.
Read the full abstract

Classical Analysis and ODEs

Sharp $\ell^p$-Improving Estimates for Fixed-Radius Discrete Spherical Averages

Let $d\geq 4$ and let $R>0$. When $d=4$, assume that $R^2\in\mathbb{N}\setminus 4\mathbb{N}$; when $d\geq 5$, let $R^2\in\mathbb{N}$ be arbitrary. We prove the fixed-radius estimate $$\|A_R f\|_{\ell^{p'}(\mathbb{Z}^d)}\leq C_{d,p,\varepsilon}R^{-d(2/p-1)+\varepsilon}\|f\|_{\ell^p(\mathbb{Z}^d)}$$ for $(d+2)/d\leq p\leq 2$, where $p'$ is the Hölder conjugate exponent of $p$ and $A_R$ is the probability average over the lattice sphere of radius $R$. This extends the fixed-radius estimates of Kesler–Lacey and Hughes to the sharp lower endpoint $p=(d+2)/d$.
Read the full abstract

Classical Analysis and ODEs

Structure of Minimal Locally Concave Functions

We propose a theory describing the structure of minimal locally concave functions on an arbitrary subdomain of $\mathbb{R}^d$. We introduce the notion of an extremal set, that is, a set on which the function is affine, and prove that extremal sets foliate the domain. We show that extremal sets are covered by simplices with vertices on the boundary of the domain, and that these simplices themselves lie in the extremal set. We also prove that one-dimensional extremal sets can approach the free boundary only tangentially, not transversally.
Read the full abstract

Classical Analysis and ODEs

Sharp $L^2$ Estimates for $(2+1)$-dimensional oscillatory integral operators with homogeneous binomial phases

We study oscillatory integral operators in $(2+1)$-dimensions with a homogeneous binomial phase \[ \Phi(x,y,t)=x^{k-k_P}t^{k_P}+y^{k-k_Q}t^{k_Q}, \qquad 1\le k_P<k_Q<k. \] For compactly supported smooth amplitudes, we establish sharp \(L^2(\R)\to L^2(\R^2)\) estimates with logarithmic losses occurring only in certain critical cases. The proof is based on scale-dependent Phong–Stein estimates.
Read the full abstract

Classical Analysis and ODEs

Generalized Freud weight, discrete Painlevé I hierarchy and full asymptotics of Hankel determinants

In this paper, we investigate the monic orthogonal polynomials $P_{n}(x;T_{m};\lambda)$ and the Hankel determinants $D_{n}(T_{m}; \lambda)$ associated with the generalized Freud weight \[w(x;T_{m};\lambda) = |x|^{2\lambda+1}\exp\biggl(-\sum_{k=1}^m t_k x^{2k}\biggr),\quad m \in \mathbb{Z}^+,\; t_{k} \in \mathbb{R} , x\in\mathbb{R}\setminus\{0\},\] where \(T_{m}=\{t_{1},t_{2},\cdots, t_{m}\}\), $t_{m}>0$ and \(\lambda>-1\).By employing ladder operators and compatibility conditions, we find that all members of the discrete Painlevé I hierarchy have a unified structure and the recurrence coefficient \(\beta_n\) of $P_{n}(x;T_{m};\lambda)$ satisfies the $m$-th member of the discrete Painlevé I hierarchy. Besides, we derive the second-order differential equation satisfied by $P_{n}(x;T_{m};\lambda)$, the partial derivatives of the recurrence coefficients \(\beta_n\) with respect to parameters \(t_1, t_2, \dots, t_{m-1}\) and the corresponding differential identities for $D_{n}(T_{m}; \lambda)$. Based on the discrete Painlevé I hierarchy and the above differential identities, we obtain new partial differential equations satisfied by $\ln\beta_n$ and $\ln D_{n}(T_{m}; \lambda)$.Using the discrete Painlevé I hierarchy and the asymptotic theory of linear difference equations, we derive the full asymptotic expansions of the recurrence coefficient $\beta_n$, the nontrivial leading coefficient $\mathrm{p}(n; T_m; \lambda)$, and the Hankel determinant $D_n(T_m; \lambda)$ as $n\to\infty$, for general $T_m$ and $\lambda>-1$. Notably, while the logarithmic term $\ln n$ appears in the leading-order contributions, it is absent from the remainder terms in these expansions.We illustrate our results under the specific decic Freud weight $w(x;t_1,t_2;\lambda)=|x|^{2\lambda+1} \exp\bigl(-x^{10}-t_2x^4-t_1x^2\bigr)$.
Read the full abstract