arXiv subjects

Szilárd Gy. Révész

Publications and source records attributed to Szilárd Gy. Révész.

36 records · Page 2Linked to original sources

arXiv2021

A Riemann-von Mangoldt-type formula for the distribution of Beurling primes

In this paper we work out a Riemann-von Mangoldt type formula for the summatory function $ψ(x):=\sum_{g\in G, |g|\le x} Λ_{G}(g)$, where $G$ is an arithmetical semigroup (a Beurling generalized system of integers) and $Λ_{G}$ is the corresponding von Mangoldt function attaining $\log|p|$ for $g=p^k$ with a prime element $p\in G$ and zero otherwise. On the way towards this formula, we prove explicit estimates on the Beurling zeta function $ζ_{G}$, belonging to $G$, to the number of zeroes of $ζ_G$ in various regions, in particular within the critical strip where the analytic continuation exists, and to the magnitude of the logarithmic derivative of $ζ_G$, under the sole additional assumption that Knopfmacher's Axiom A is satisfied. We also construct a technically useful broken line contour to which the technic of integral transformation can be well applied. The whole work serves as a first step towards a further study of the distribution of zeros of the Beurling zeta function, providing appropriate zero density and zero clustering estimates, to be presented in the continuation of this paper.

math.NT↗

arXiv2019

Integral comparisons of nonnegative positive definite functions on LCA groups

In this paper we investigate the following questions. Let $μ, ν$ be two regular Borel measures of finite total variation. When do we have a constant $C$ satisfying $$\int f dν\le C \int f dμ$$ whenever $f$ is a continuous nonnegative positive definite function? How the admissible constants $C$ can be characterized, and what is their optimal value? We first discuss the problem in locally compact abelian groups. Then we make further specializations when the Borel measures $μ, ν$ are both either purely atomic or absolutely continuous with respect to a reference Haar measure. In addition, we prove a duality conjecture posed in our former paper.

math.FA↗

arXiv2018

Turán-Erőd type converse Markov inequalities on general convex domains of the plane in $\boldsymbol{L^q}$

In 1939 P. Turán started to derive lower estimations on the norm of the derivatives of polynomials of (maximum) norm 1 on $I:=[-1,1]$ (interval) and $D:=\{z\in\mathbb{C}~:~|z|\le 1\}$ (disk), under the normalization condition that the zeroes of the polynomial in question all lie in $I$ or $D$, respectively. For the maximum norm he found that with $n:=\mathop{\mathrm{deg}} p$ tending to infinity, the precise growth order of the minimal possible derivative norm is $\sqrt{n}$ for $I$ and $n$ for $D$. J. Erőd continued the work of Turán considering other domains. Finally, a decade ago the growth of the minimal possible $\infty$-norm of the derivative was proved to be of order $n$ for all compact convex domains. Although Turán himself gave comments about the above oscillation question in $L^q$ norms, till recently results were known only for $D$ and $I$. Recently, we have found order $n$ lower estimations for several general classes of compact convex domains, and conjectured that even for arbitrary convex domains the growth order of this quantity should be $n$. Now we prove that in $L^q$ norm the oscillation order is at least $n/\log n$ for all compact convex domains.

math.CA↗

arXiv2017

A potential theoretic minimax problem on the torus

We investigate an extension of an equilibrium-type result, conjectured by Ambrus, Ball and Erdélyi, and proved recently by Hardin, Kendall and Saff. These results were formulated on the torus, hence we also work on the torus, but one of the main motivations for our extension comes from an analogous setup on the unit interval, investigated earlier by Fenton. Basically, the problem is a minimax one, i.e. to minimize the maximum of a function $F$, defined as the sum of arbitrary translates of certain fixed "kernel functions", minimization understood with respect to the translates. If these kernels are assumed to be concave, having certain singularities or cusps at zero, then translates by $y_j$ will have singularities at $y_j$ (while in between these nodes the sum function still behaves realtively regularly). So one can consider the maxima $m_i$ on each subintervals between the nodes $y_j$, and look for the minimization of $\max F = \max_i m_i$. Here also a dual question of maximization of $\min_i m_i$ arises. This type of minimax problems were treated under some additional assumptions on the kernels. Also the problem is normalized so that $y_0=0$. In particular, Hardin, Kendall and Saff assumed that we have one single kernel $K$ on the torus or circle, and $F=\sum_{j=0}^n K(\cdot-y_j)= K + \sum_{j=1}^n K(\cdot-y_j)$. Fenton considered situations on the interval with two fixed kernels $J$ and $K$, also satisfying additional assumptions, and $F= J + \sum_{j=1}^n K(\cdot-y_j)$. Here we consider the situation (on the circle) when \emph{all the kernel functions can be different}, and $F=\sum_{j=0}^n K_j(\cdot- y_j) = K_0 + \sum_{j=1}^n K_j(\cdot-y_j)$. Also an emphasis is put on relaxing all other technical assumptions and give alternative, rather minimal variants of the set of conditions on the kernel.

math.CA↗

arXiv2016

Turán type converse Markov inequalities in $L^q$ on a generalized Erőd class of convex domains

P. Turán was the first to derive lower estimations on the uniform norm of the derivatives of polynomials $p$ of uniform norm $1$ on the interval I:=[-1,1] and the disk D:=$\{z \in C~:~|z| \le 1\}$, under the normalization condition that the zeroes of the polynomial p in question all lie in I or D, resp. Namely, in 1939 he proved that with n:=deg p tending to infinity, the precise growth order of the minimal possible derivative norm is $\sqrt{n}$ for I and n for D. Already the same year J. Erőd considered the problem on other domains. In his most general formulation, he extended Turán's order n result on D to a certain general class of piecewise smooth convex domains. Finally, a decade ago the growth order of the minimal possible norm of the derivative was proved to be n for all compact convex domains. Turán himself gave comments about the above oscillation question in $L^q$ norm on D. Nevertheless, till recently results were known only for I, D and so-called R-circular domains. Continuing our recent work, also here we investigate the Turán-Erőd problem on general classes of domains.

math.CV↗

arXiv2015

Turán type oscillation inequalities in $L^q$ norm on the boundary of convex domains

Some 76 years ago P. Turán was the first to establish lower estimations of the ratio of the maximum norm of the derivatives of polynomials and the maximum norm of the polynomials themselves on the interval I:=[-1,1] and on the unit disk D:={z : |z| <= 1} under the normalization condition that the zeroes of the polynomial all lie in the interval or in the disk, respectively. He proved that with n:=deg p tending to infinity, the precise growth order of the minimal possible ratio of the derivative norm and the norm is square-root{n} for I and n for D. J. Eröd continued the work of Turán and extended his results to several other domains. The growth of the minimal possible ratio of the infinity norm of the derivative and the polynomial itself was proved to be of order n for all compact convex domains a decade ago. Although Turán himself gave comments about the above oscillation question in Lq norms, till recently results were known only for D and I. Here we prove that in Lq norm the oscillation order is again n for a certain class of convex domains, including all smooth convex domains and also convex polygonal domains having no acute angles at their vertices.

math.CA↗

arXiv2015

The point value maximization problem for positive definite functions supported in a given subset of a locally compact group

The century old extremal problem, solved by Carathéodory and Fejér, concerns a nonnegative trigonometric polynomial normalized by a0 = 1, and the quantity to be maximized is the coefficient a1. In the complex exponential form, the coefficient sequence (ck) will be supported in [-n; n] and normalized by c0 =1. Reformulating, nonnegativity of T translates to positive definiteness of the sequence (ck), and the extremal problem becomes a maximization problem for the value at 1 of a normalized positive definite function c : Z --> C, supported in [-n; n]. Boas and Katz, Arestov, Berdysheva and Berens, Kolountzakis and Révész and recently Krenedits and Révész investigated the problem in increasing generality, reaching analogous results for all locally compact Abelian groups. We prove an extension to all the known results in not necessarily commutative locally compact groups.

math.AP↗

arXiv2013

The Caratheodory-Fejér type extremal problem on locally compact Abelian groups

We consider the extremal problem of maximizing a point value jf(z)j at a given point z 2 G by some positive definite and continuous function f on an Abelian group G, where for a given symmetric open set 3 z, f vanishes outside and is normalized by f(0) = 1. Denote the extremal value as CG(; z). This extremal problem was investigated in R and Rd and for a 0-symmetric convex body in a paper of Boas and Kac in 1943. Arestov and Berdysheva extended the investigation to Td, where T := R=Z. Kolountzakis and R?ev?esz gave a more general setting, considering arbitrary open sets, in all the classical groups above. Also they observed, that such extremal problems occurred in certain special cases and in a different, but equivalent formulation already a century ago in the work of Carath?eodory and Fej?er. Moreover, following observations of Boas and Kac, Kolountzakis and R?ev?esz showed how the general problem can be reduced to equivalent discrete problems of "Carath?eodory-Fej?er type" on Z or Zm := Z=mZ. We extend the results of Kolountzakis and R?ev?esz to locally compact Abelian groups.

math.AP↗

arXiv2012

Generalization of the effective Wiener-Ikehara theorem

We consider the classical Wiener-Ikehara Tauberian theorem, with a generalized condition of slow decrease and some additional poles on the boundary of convergence of the Laplace transform. In this generality, we prove the otherwise known asymptotic evaluation of the transformed function, when the usual conditions of the Wiener-Ikehara theorem hold. However, our version also provides an effective error term, not known thus far in this generalality. The crux of the proof is a proper, asymptotic variation of the lemmas of Ganelius and Tenenbaum, also constructed for the sake of an effective version of the Wiener-Ikehara Theorem.

math.NT↗

arXiv2008

Concentration of the integral norm of idempotents

This is a companion paper of a recent one, entitled {\sl Integral concentration of idempotent trigonometric polynomials with gaps}. New results of the present work concern $L^1$ concentration, while the above mentioned paper deals with $L^p$-concentration. Our aim here is two-fold. At the first place we try to explain methods and results, and give further straightforward corollaries. On the other hand, we push forward the methods to obtain a better constant for the possible concentration (in $L^1$ norm) of an idempotent on an arbitrary symmetric measurable set of positive measure. We prove a rather high level $γ_1>0.96$, which contradicts strongly the conjecture of Anderson et al. that there is no positive concentration in $L^1$ norm. The same problem is considered on the group $\mathbb{Z}/q\mathbb{Z}$, with $q$ say a prime number. There, the property of absolute integral concentration of idempotent polynomials fails, which is in a way a positive answer to the conjecture mentioned above. Our proof uses recent results of B. Green and S. Konyagin on the Littlewood Problem.

math.CA↗

arXiv2008

Integral Concentration of idempotent trigonometric polynomials with gaps

We prove that for all p>1/2 there exists a constant $γ_p>0$ such that, for any symmetric measurable set of positive measure $E\subset \TT$ and for any $γ<γ_p$, there is an idempotent trigonometrical polynomial f satisfying $\int_E |f|^p > γ\int_{\TT} |f|^p$. This disproves a conjecture of Anderson, Ash, Jones, Rider and Saffari, who proved the existence of $γ_p>0$ for p>1 and conjectured that it does not exists for p=1. Furthermore, we prove that one can take $γ_p=1$ when p>1 is not an even integer, and that polynomials f can be chosen with arbitrarily large gaps when $p\neq 2$. This shows striking differences with the case p=2, for which the best constant is strictly smaller than 1/2, as it has been known for twenty years, and for which having arbitrarily large gaps with such concentration of the integral is not possible, according to a classical theorem of Wiener. We find sharper results for $0<p\leq 1$ when we restrict to open sets, or when we enlarge the class of idempotent trigonometric polynomials to all positive definite ones.

math.CA↗

arXiv2007

Failure of Wiener's property for positive definite periodic functions

We say that Wiener's property holds for the exponent $p>0$ if we have that whenever a positive definite function $f$ belongs to $L^p(-ε,ε)$ for some $ε>0$, then $f$ necessarily belongs to $L^p(\TT)$, too. This holds true for $p\in 2\NN$ by a classical result of Wiener. Recently various concentration results were proved for idempotents and positive definite functions on measurable sets on the torus. These new results enable us to prove a sharp version of the failure of Wiener's property for $p\notin 2\NN$. Thus we obtain strong extensions of results of Wainger and Shapiro, who proved the negative answer to Wiener's problem for $p\notin 2\NN$.

math.CA↗

arXiv2007

Equivalence of A-Approximate Continuity for Self-Adjoint Expansive Linear Maps

Let A be an expansive linear map from R^d to R^d. The notion of A-approximate continuity was recently used to give a characterization of scaling functions in a multiresolution analysis (MRA). The definition of A-approximate continuity at a point x - or, equivalently, the definition of the family of sets having x as point of A-density - depend on the expansive linear map A. The aim of the present paper is to characterize those self-adjoint expansive linear maps A_1, A_2 for which the respective concepts of A_j-approximate continuity (j=1,2) coincide. These we apply to analyze the equivalence among dilation matrices for a construction of systems of MRA. In particular, we give a full description for the equivalence class of the dyadic dilation matrix among all self-adjoint expansive maps. If the so-called ``four exponentials conjecture'' of algebraic number theory holds true, then a similar full description follows even for general self-adjoint expansive linear maps, too.

math.CA↗

arXiv2007

Positive bases in spaces of polynomials

For a nonempty compact set D of R we determine the maximal possible dimension of a subspace X of polynomial functions of degree at most m which possesses a positive bases (where positivity is understood on D). The exact value of this maximal dimension depends on topological features of the base set D. We show that at many cases dimension m can be achieved.Whereas only for low m or finite sets D it is possible to have m+1 (the full dimension of the space of polynomials of degree at most m) dimensional subspace X with positive basis. Therefore, it turns out that for no D it is possible to have a positive basis of the polynomial space of degree at most m for all m in N.

math.CA↗

arXiv2007

Invariant decomposition of functions with respect to commuting invertible transformations

Consider a_1,a_2,...,a_n, arbitrary elements of R. We characterize those real functions f that decompose into the sum of a_j-periodic functions, i.e., f=f_1+...+f_n with D_{a_j}f(x):=f(x+a_j)-f(x)=0. We show that f has such a decomposition if and only if for all partitions to B_1, B_2,... B_N of {a_1,a_2,...,a_n} with B_j consisting of commensurable elements with least common multiples b_j, one has D_{b_1}... D_{b_N}f=0. Actually, we prove a more general result for periodic decompositions of real functions f defined on an Abelian group A, and, in fact, we even consider invariant decompositions of functions f defined on some abstract set A, with respect to commuting, invertible self-mappings of the set A. We also extend our results to functions between torsion free Abelian groups. As a corollary we also obtain that on a torsion free Abelian group the existence of a real valued periodic decomposition of an integer valued function implies the existence of an integer valued periodic decomposition with the same periods.

math.CA↗

arXiv2007

On a paper of Erod and Turan-Markov inequalities for non-flat convex domains

For a convex domain K in the complex plane C, the well-known general Markov inequality asserting that a polynomial p of degree n ||p'|| < c(K) n^2 ||p|| holds. On the other hand for polynomials in general, ||p'|| can be arbitrarily small as compared to ||p||. The situation changes when we assume that the polynomials have all their zeroes in the convex body K. This problem of lower bound for Markov factors was first investigated by Turán in 1939. Turán showed ||p'|| \ge n/2 ||p|| for the unit disk D and ||p'|| > c \sqrt{n} ||p|| for the unit interval I:=[-1,1]. Soon after that, J. Er\H od published a long article, discussing various extensions of the results and methods of Turán. For decades, Er\H od's paper was quoted only for the explicit calculation of the exact constant of the interval case. However, in recent years Levenberg and Poletsky, Erdélyi and also the author investigated Turán's problem for various sets - basically, convex domains. In this context the much richer content of Er\H od's work is to be realized again. Thus, the aim of the paper is twofold. On the one hand we give an account of the half-forgotten, old Hungarian article of Er\H od, also commemorating its author. On the other hand we report on recent developments with particular emphasis on development of one of the key observations of Er\H od, namely, the role of the curvature of the boundary curve in the estimation of the lower bound of Markov factors.

math.CA↗

arXiv2007

Inequalities for Multivariate Polynomials

We summarize researches - in great deal jointly with my host Y. Sarantopoulos and his PhD. students V. Anagnostopoulos and A. Pappas - started by a Marie Curie fellowship in 2001 and is still continuing. The project was to study multivariate polynomial inequalities. In the course of work we realized the role of the ``generalized Minkowski functional'', to which we devoted a throughout survey. Resulting from this, infinite dimensional extensions of Chebyshev's extremal problems were tackled successfully. Investigating Bernstein-Markov constants for homogeneous polynomials of real normed spaces led us to the application of potential theory. Also we found at first unexpected connections of polarization constants of R^2 and C^2 to Chebyshev constants of S^2 and S^3, respectively. In the study of polarization constants, a further application of potential theory occurred. This led us to realize that the theory of rendezvous numbers can be much better explained by potential theory, too. Our methods for obtaining Bernstein type pointwise gradient estimates for polynomials were compared in a recent case study to the yields of pluripotential theoretic methods. The findings were that the two rather different methods give exactly the same results, but the two currently standing conjectures mutually exclude each other.

math.CA↗

arXiv2007

Schur type inequalities for complex polynomials with no zeros in the unit disk

Starting out from a question posed by T. Erdélyi and J. Szabados, we consider Schur-type inequalities for the classes of complex algebraic polynomials having no zeroes within the unit disk D. The class of polynomials with no zeroes in D - also known as Bernstein- or Lorentz-class - was studied in detail earlier. For real polynomials utilizing the Bernstein-Lorentz representation as convex combinations of fundamental polynomials (1-x)^k(1+x)^{n-k}, G. Lorentz, T. Erdélyi and J. Szabados proved a number of improved versions of Schur- (and also Bernstein- and Markov-) type inequalities. Here we investigate the similar questions for complex polynomials. For complex polynomials the above convex representation is not available. Even worse, the set of complex polynomials, having no zeroes in the unit disk, does not form a convex set. Therefore, a possible proof must go along different lines. In fact such a direct argument was asked for by Erdélyi and Szabados already for the real case. The sharp forms of the Bernstein- and Markov- type inequalities are known, and the right factors are worse for complex coefficients than for real ones. However, here it turns out that Schur-type inequalities hold unchanged even for complex polynomials and for all monotonic, continuous weight functions. As a consequence, it becomes possible to deduce the corresponding Markov inequality from the known Bernstein inequality and the new Schur type inequality with logarithmic weight.

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