Pub Date : 2025-08-07DOI: 10.1016/j.jat.2025.106220
D. Leviatan , I. Shevchuk , V. Shevchuk
Let . For a continuous -periodic function, changing its monotonicity times in a period, and whose degree of approximation by trigonometric polynomials of degree , is , , we investigate its degree of approximation by such polynomials that, in addition, follow the changes of monotonicity. Obviously, the unconstrained degree is smaller than the constrained one, but for , there is a constant such that the constrained degree is , . On the other hand we show that, in general, this is invalid for .
{"title":"Comparing the degree of constrained and unconstrained trigonometric approximation","authors":"D. Leviatan , I. Shevchuk , V. Shevchuk","doi":"10.1016/j.jat.2025.106220","DOIUrl":"10.1016/j.jat.2025.106220","url":null,"abstract":"<div><div>Let <span><math><mrow><mi>r</mi><mo>,</mo><mi>s</mi><mo>∈</mo><mi>N</mi></mrow></math></span>. For a continuous <span><math><mrow><mn>2</mn><mi>π</mi></mrow></math></span>-periodic function, changing its monotonicity <span><math><mrow><mn>2</mn><mi>s</mi></mrow></math></span> times in a period, and whose degree of approximation by trigonometric polynomials of degree <span><math><mrow><mo><</mo><mi>n</mi></mrow></math></span>, is <span><math><mrow><mo>≤</mo><msup><mrow><mi>n</mi></mrow><mrow><mo>−</mo><mi>r</mi></mrow></msup></mrow></math></span>, <span><math><mrow><mi>n</mi><mo>≥</mo><mn>1</mn></mrow></math></span>, we investigate its degree of approximation by such polynomials that, in addition, follow the changes of monotonicity. Obviously, the unconstrained degree is smaller than the constrained one, but for <span><math><mrow><mi>r</mi><mo>></mo><mn>2</mn><mi>s</mi><mo>−</mo><mn>2</mn></mrow></math></span>, there is a constant <span><math><mrow><mi>c</mi><mrow><mo>(</mo><mi>s</mi><mo>,</mo><mi>r</mi><mo>)</mo></mrow></mrow></math></span> such that the constrained degree is <span><math><mrow><mo>≤</mo><mi>c</mi><mrow><mo>(</mo><mi>s</mi><mo>,</mo><mi>r</mi><mo>)</mo></mrow><msup><mrow><mi>n</mi></mrow><mrow><mo>−</mo><mi>r</mi></mrow></msup></mrow></math></span>, <span><math><mrow><mi>n</mi><mo>≥</mo><mn>1</mn></mrow></math></span>. On the other hand we show that, in general, this is invalid for <span><math><mrow><mi>r</mi><mo>≤</mo><mn>2</mn><mi>s</mi><mo>−</mo><mn>2</mn></mrow></math></span>.</div></div>","PeriodicalId":54878,"journal":{"name":"Journal of Approximation Theory","volume":"312 ","pages":"Article 106220"},"PeriodicalIF":0.6,"publicationDate":"2025-08-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144809532","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}