{"title":"Indefinite Sturm–Liouville Operators in Polar Form","authors":"Branko Ćurgus, Volodymyr Derkach, Carsten Trunk","doi":"10.1007/s00020-023-02746-3","DOIUrl":"https://doi.org/10.1007/s00020-023-02746-3","url":null,"abstract":"<p>We consider the indefinite Sturm–Liouville differential expression </p><span>$$begin{aligned} {mathfrak {a}}(f):= - frac{1}{w}left( frac{1}{r} f' right) ', end{aligned}$$</span><p>where <span>({mathfrak {a}})</span> is defined on a finite or infinite open interval <i>I</i> with <span>(0in I)</span> and the coefficients <i>r</i> and <i>w</i> are locally summable and such that <i>r</i>(<i>x</i>) and <span>(({text {sgn}},x) w(x))</span> are positive a.e. on <i>I</i>. With the differential expression <span>({mathfrak {a}})</span> we associate a nonnegative self-adjoint operator <i>A</i> in the Krein space <span>(L^2_w(I))</span> which is viewed as a coupling of symmetric operators in Hilbert spaces related to the intersections of <i>I</i> with the positive and the negative semi-axis. For the operator <i>A</i> we derive conditions in terms of the coefficients <i>w</i> and <i>r</i> for the existence of a Riesz basis consisting of generalized eigenfunctions of <i>A</i> and for the similarity of <i>A</i> to a self-adjoint operator in a Hilbert space <span>(L^2_{|w|}(I))</span>. These results are obtained as consequences of abstract results about the regularity of critical points of nonnegative self-adjoint operators in Krein spaces which are couplings of two symmetric operators acting in Hilbert spaces.</p>","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"10 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139552743","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":"Some Operator Ideal Properties of Volterra Operators on Bergman and Bloch Spaces","authors":"Joelle Jreis, Pascal Lefèvre","doi":"10.1007/s00020-023-02742-7","DOIUrl":"https://doi.org/10.1007/s00020-023-02742-7","url":null,"abstract":"<p>We characterize the integration operators <span>(V_g)</span> with symbol <i>g</i> for which <span>(V_g)</span> acts as an absolutely summing operator on weighted Bloch spaces <span>(mathcal {B}^{beta })</span> and on weighted Bergman spaces <span>(mathscr {A}^p_alpha )</span>. We show that <span>(V_g)</span> is <i>r</i>-summing on <span>(mathscr {A}^p_alpha )</span>, <span>(1 le p <infty )</span>, if and only if <i>g</i> belongs to a suitable Besov space. We also show that there is no non trivial nuclear Volterra operators <span>(V_g)</span> on Bloch spaces and on Bergman spaces.</p>","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"7 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138560805","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":"Phases of Sectorial Operators","authors":"Tianqiu Yu, Di Zhao, Li Qiu","doi":"10.1007/s00020-023-02752-5","DOIUrl":"https://doi.org/10.1007/s00020-023-02752-5","url":null,"abstract":"<p>In this paper, we first define the phases of a sectorial operator based on the numerical range. We are interested in the estimation of the spectrum phases of <i>AB</i> based on the phases of two sectorial operators <i>A</i> and <i>B</i>. Motivated by the classical small gain theorem, we formulate an operator small phase theorem with necessity for the invertibility of <span>(I+AB)</span>, which plays a crucial role in feedback stability analysis. Afterwards, we consider the special class of sectorial operators of the form <span>(P+K)</span>, where <i>P</i> is strictly positive and <i>K</i> is compact. More properties of the phases for those operators are studied, including those of compressions, Schur complements, operator means and products. Finally, for the special class of sectorial operators, we further establish a majorization relation between the phases of the spectrum of <i>AB</i> and the phases of two operators <i>A</i> and <i>B</i>.</p>","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"27 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2023-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138518210","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":"Denjoy–Wolff Points on the Bidisk via Models","authors":"Michael T. Jury, Georgios Tsikalas","doi":"10.1007/s00020-023-02751-6","DOIUrl":"https://doi.org/10.1007/s00020-023-02751-6","url":null,"abstract":"","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"50 8","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134902650","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":"Stability Analysis of a Simple Discretization Method for a Class of Strongly Singular Integral Equations","authors":"Martin Costabel, Monique Dauge, Khadijeh Nedaiasl","doi":"10.1007/s00020-023-02750-7","DOIUrl":"https://doi.org/10.1007/s00020-023-02750-7","url":null,"abstract":"","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"49 5","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135037590","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":"Decay Rate of $$varvec{exp (A^{-1}t)A^{-1}}$$ on a Hilbert Space and the Crank–Nicolson Scheme with Smooth Initial Data","authors":"Masashi Wakaiki","doi":"10.1007/s00020-023-02748-1","DOIUrl":"https://doi.org/10.1007/s00020-023-02748-1","url":null,"abstract":"Abstract This paper is concerned with the decay rate of $$e^{A^{-1}t}A^{-1}$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:msup> <mml:mi>e</mml:mi> <mml:mrow> <mml:msup> <mml:mi>A</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mi>t</mml:mi> </mml:mrow> </mml:msup> <mml:msup> <mml:mi>A</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> for the generator A of an exponentially stable $$C_0$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:msub> <mml:mi>C</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:math> -semigroup on a Hilbert space. To estimate the decay rate of $$e^{A^{-1}t}A^{-1}$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:msup> <mml:mi>e</mml:mi> <mml:mrow> <mml:msup> <mml:mi>A</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mi>t</mml:mi> </mml:mrow> </mml:msup> <mml:msup> <mml:mi>A</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , we apply a bounded functional calculus. Using this estimate and Lyapunov equations, we also study the quantified asymptotic behavior of the Crank-Nicolson scheme with smooth initial data. A similar argument is applied to a polynomially stable $$C_0$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:msub> <mml:mi>C</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:math> -semigroup whose generator is normal.","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"26 12","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135037286","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":"Heat Flow in Polygons with Reflecting Edges","authors":"Sam Farrington, Katie Gittins","doi":"10.1007/s00020-023-02749-0","DOIUrl":"https://doi.org/10.1007/s00020-023-02749-0","url":null,"abstract":"Abstract We investigate the heat flow in an open, bounded set D in $$mathbb {R}^2$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:msup> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:math> with polygonal boundary $$partial D$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:mi>∂</mml:mi> <mml:mi>D</mml:mi> </mml:mrow> </mml:math> . We suppose that D contains an open, bounded set $$widetilde{D}$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>~</mml:mo> </mml:mover> </mml:math> with polygonal boundary $$partial widetilde{D}$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:mi>∂</mml:mi> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>~</mml:mo> </mml:mover> </mml:mrow> </mml:math> . The initial condition is the indicator function of $$widetilde{D}$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>~</mml:mo> </mml:mover> </mml:math> and we impose a Neumann boundary condition on the edges of $$partial D$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:mi>∂</mml:mi> <mml:mi>D</mml:mi> </mml:mrow> </mml:math> . We obtain an asymptotic formula for the heat content of $$widetilde{D}$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mover> <mml:mi>D</mml:mi> <mml:mo>~</mml:mo> </mml:mover> </mml:math> in D as time $$tdownarrow 0$$ <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\"> <mml:mrow> <mml:mi>t</mml:mi> <mml:mo>↓</mml:mo> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> .","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"316 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135474920","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":"Dyadic Maximal Operators on Martingale Musielak–Orlicz Hardy Type Spaces and Applications","authors":"Weisz Ferenc, Guangheng Xie, Dachun Yang","doi":"10.1007/s00020-023-02747-2","DOIUrl":"https://doi.org/10.1007/s00020-023-02747-2","url":null,"abstract":"","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"63 5","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135476598","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":"On the Power Set of Quasinilpotent Operators","authors":"Youqing Ji, Yuanhang Zhang","doi":"10.1007/s00020-023-02745-4","DOIUrl":"https://doi.org/10.1007/s00020-023-02745-4","url":null,"abstract":"","PeriodicalId":13658,"journal":{"name":"Integral Equations and Operator Theory","volume":"55 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135970084","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}