{"title":"Littlewood–Paley–Rubio de Francia inequality for unbounded Vilenkin systems","authors":"Anton Tselishchev","doi":"10.1016/j.jat.2023.106006","DOIUrl":"https://doi.org/10.1016/j.jat.2023.106006","url":null,"abstract":"<div><p>Rubio de Francia proved the one-sided version of Littlewood–Paley inequality for arbitrary intervals. In this paper, we prove the similar inequality in the context of arbitrary Vilenkin systems (that is, for functions on infinite products of cyclic groups). There are no assumptions on the orders of these groups.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"298 ","pages":"Article 106006"},"PeriodicalIF":0.9,"publicationDate":"2023-11-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138466319","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"The sharp Landau–Kolmogorov inequality for the set ‖y′‖2, ‖y‖1, ‖y+′′‖∞ on the real line","authors":"N.S. Payuchenko","doi":"10.1016/j.jat.2023.105996","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105996","url":null,"abstract":"<div><p>We obtain the sharp Kolmogorov inequality <span><math><mrow><msub><mrow><mo>‖</mo><msup><mrow><mi>y</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>‖</mo></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mn>2</mn></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow></msub><mo>≤</mo><mfrac><mrow><mn>2</mn><msqrt><mrow><mn>2</mn></mrow></msqrt></mrow><mrow><msqrt><mrow><mn>3</mn></mrow></msqrt></mrow></mfrac><msubsup><mrow><mo>‖</mo><mi>y</mi><mo>‖</mo></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mn>1</mn></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msubsup><msubsup><mrow><mo>‖</mo><msubsup><mrow><mi>y</mi></mrow><mrow><mo>+</mo></mrow><mrow><mo>′</mo><mo>′</mo></mrow></msubsup><mo>‖</mo></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mi>∞</mi></mrow></msub><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msubsup></mrow></math></span> on the real line <span><math><mrow><mi>G</mi><mo>=</mo><mi>R</mi></mrow></math></span> and the period <span><math><mrow><mi>G</mi><mo>=</mo><mrow><mo>[</mo><mn>0</mn><mo>,</mo><mn>1</mn><mo>)</mo></mrow></mrow></math></span>.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"298 ","pages":"Article 105996"},"PeriodicalIF":0.9,"publicationDate":"2023-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138435954","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Coefficient-based regularized distribution regression","authors":"Yuan Mao , Lei Shi , Zheng-Chu Guo","doi":"10.1016/j.jat.2023.105995","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105995","url":null,"abstract":"<div><p>In this paper, we consider the coefficient-based regularized distribution regression which aims to regress from probability measures to real-valued responses over a reproducing kernel Hilbert space (RKHS), where the regularization is put on the coefficients and kernels are assumed to be indefinite. The algorithm involves two stages of sampling, the first stage sample consists of distributions and the second stage sample is obtained from these distributions. The asymptotic behavior of the algorithm is comprehensively studied across different regularity ranges of the regression function. Explicit learning rates are derived by using kernel mean embedding and integral operator techniques. We obtain the optimal rates under some mild conditions, which match the one-stage sampled minimax optimal rate. Compared with the kernel methods for distribution regression in existing literature, the algorithm under consideration does not require the kernel to be symmetric or positive semi-definite and hence provides a simple paradigm for designing indefinite kernel methods, which enriches the theme of the distribution regression. To the best of our knowledge, this is the first result for distribution regression with indefinite kernels, and our algorithm can improve the learning performance against saturation effect.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"297 ","pages":"Article 105995"},"PeriodicalIF":0.9,"publicationDate":"2023-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"92045503","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Triebel–Lizorkin regularity and bi-Lipschitz maps: Composition operator and inverse function regularity","authors":"Martí Prats","doi":"10.1016/j.jat.2023.105985","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105985","url":null,"abstract":"<div><p><span>We study the stability of Triebel–Lizorkin regularity of bounded functions and </span>Lipschitz functions<span> under bi-Lipschitz changes of variables and the regularity of the inverse function<span> of a Triebel–Lizorkin bi-Lipschitz map in Lipschitz domains. To obtain the results we provide an equivalent norm for the Triebel–Lizorkin spaces with fractional smoothness in uniform domains in terms of the first-order difference of the last weak derivative available averaged on balls.</span></span></p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"297 ","pages":"Article 105985"},"PeriodicalIF":0.9,"publicationDate":"2023-10-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"92045502","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Manuel Bello-Hernández , Alejandro del Campo López
{"title":"Mean convergence of Fourier–Akhiezer–Chebyshev series","authors":"Manuel Bello-Hernández , Alejandro del Campo López","doi":"10.1016/j.jat.2023.105984","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105984","url":null,"abstract":"<div><p>We prove mean convergence of the Fourier series in Akhiezer–Chebyshev polynomials in <span><math><msup><mrow><mi>L</mi></mrow><mrow><mi>p</mi></mrow></msup></math></span>, <span><math><mrow><mi>p</mi><mo>></mo><mn>1</mn></mrow></math></span>, using a weighted inequality for the Hilbert transform in an arc of the unit circle.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"296 ","pages":"Article 105984"},"PeriodicalIF":0.9,"publicationDate":"2023-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"50191862","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Vladimir Andrievskii, András Kroó, József Szabados
{"title":"Richard S. Varga October 9, 1928 – February 25, 2022","authors":"Vladimir Andrievskii, András Kroó, József Szabados","doi":"10.1016/j.jat.2023.105971","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105971","url":null,"abstract":"","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"297 ","pages":"Article 105971"},"PeriodicalIF":0.9,"publicationDate":"2023-10-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"50180386","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Discrete harmonic analysis associated with Jacobi expansions III: The Littlewood–Paley–Stein gk-functions and the Laplace type multipliers","authors":"Alberto Arenas, Óscar Ciaurri, Edgar Labarga","doi":"10.1016/j.jat.2023.105940","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105940","url":null,"abstract":"<div><p>The research about harmonic analysis associated with Jacobi expansions carried out in Arenas et al. (2020) and Arenas et al. (2022) is continued in this paper. Given the operator <span><math><mrow><msup><mrow><mi>J</mi></mrow><mrow><mrow><mo>(</mo><mi>α</mi><mo>,</mo><mi>β</mi><mo>)</mo></mrow></mrow></msup><mo>=</mo><msup><mrow><mi>J</mi></mrow><mrow><mrow><mo>(</mo><mi>α</mi><mo>,</mo><mi>β</mi><mo>)</mo></mrow></mrow></msup><mo>−</mo><mi>I</mi></mrow></math></span>, where <span><math><msup><mrow><mi>J</mi></mrow><mrow><mrow><mo>(</mo><mi>α</mi><mo>,</mo><mi>β</mi><mo>)</mo></mrow></mrow></msup></math></span> is the three-term recurrence relation for the normalized Jacobi polynomials and <span><math><mi>I</mi></math></span> is the identity operator, we define the corresponding Littlewood–Paley–Stein <span><math><msubsup><mrow><mi>g</mi></mrow><mrow><mi>k</mi></mrow><mrow><mrow><mo>(</mo><mi>α</mi><mo>,</mo><mi>β</mi><mo>)</mo></mrow></mrow></msubsup></math></span>-functions associated with it and we prove an equivalence of norms with weights for them. As a consequence, we deduce a result for Laplace type multipliers.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"294 ","pages":"Article 105940"},"PeriodicalIF":0.9,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"50187773","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Explicit expressions and computational methods for the Fortet–Mourier distance of positive measures to finite weighted sums of Dirac measures","authors":"Sander C. Hille , Esmée S. Theewis","doi":"10.1016/j.jat.2023.105947","DOIUrl":"10.1016/j.jat.2023.105947","url":null,"abstract":"<div><p>Explicit expressions and computational approaches are given for the Fortet–Mourier distance between a positively weighted sum of Dirac measures on a metric space and a positive finite Borel measure. Explicit expressions are given for the distance to a single Dirac measure. For the case of a sum of several Dirac measures one needs to resort to a computational approach. In particular, two algorithms are given to compute the Fortet–Mourier norm of a molecular measure, i.e. a finite weighted sum of Dirac measures. It is discussed how one of these can be modified to allow computation of the dual bounded Lipschitz (or Dudley) norm of such measures.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"294 ","pages":"Article 105947"},"PeriodicalIF":0.9,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41466639","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"H-sets for kernel-based spaces","authors":"R. Schaback","doi":"10.1016/j.jat.2023.105942","DOIUrl":"https://doi.org/10.1016/j.jat.2023.105942","url":null,"abstract":"","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":" ","pages":""},"PeriodicalIF":0.9,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43150721","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Approximation error for neural network operators by an averaged modulus of smoothness","authors":"Danilo Costarelli","doi":"10.1016/j.jat.2023.105944","DOIUrl":"10.1016/j.jat.2023.105944","url":null,"abstract":"<div><p>In the present paper we establish estimates for the error of approximation (in the <span><math><msup><mrow><mi>L</mi></mrow><mrow><mi>p</mi></mrow></msup></math></span>-norm) achieved by neural network (NN) operators. The above estimates have been given by means of an averaged modulus of smoothness introduced by Sendov and Popov, also known with the name of <span><math><mi>τ</mi></math></span>-modulus, in case of bounded and measurable functions on the interval <span><math><mrow><mo>[</mo><mo>−</mo><mn>1</mn><mo>,</mo><mn>1</mn><mo>]</mo></mrow></math></span>. As a consequence of the above estimates, we can deduce an <span><math><msup><mrow><mi>L</mi></mrow><mrow><mi>p</mi></mrow></msup></math></span> convergence theorem for the above family of NN operators in case of functions which are bounded, measurable, and Riemann integrable on the above interval. In order to reach the above aims, we preliminarily establish a number of results; among them we can mention an estimate for the <span><math><mi>p</mi></math></span>-norm of the operators, and an asymptotic type theorem for the NN operators in case of functions belonging to Sobolev spaces.</p></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"294 ","pages":"Article 105944"},"PeriodicalIF":0.9,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44291449","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}