{"title":"Uniqueness and stability for the solution of a nonlinear least squares problem","authors":"Meng Huang, Zhiqiang Xu","doi":"10.1090/mcom/3918","DOIUrl":"https://doi.org/10.1090/mcom/3918","url":null,"abstract":"In this paper, we focus on the nonlinear least squares: $mbox{min}_{mathbf{x} in mathbb{H}^d}| |Amathbf{x}|-mathbf{b}|$ where $Ain mathbb{H}^{mtimes d}$, $mathbf{b} in mathbb{R}^m$ with $mathbb{H} in {mathbb{R},mathbb{C} }$ and consider the uniqueness and stability of solutions. Such problem arises, for instance, in phase retrieval and absolute value rectification neural networks. For the case where $mathbf{b}=|Amathbf{x}_0|$ for some $mathbf{x}_0in mathbb{H}^d$, many results have been developed to characterize the uniqueness and stability of solutions. However, for the case where $mathbf{b} neq |Amathbf{x}_0| $ for any $mathbf{x}_0in mathbb{H}^d$, there is no existing result for it to the best of our knowledge. In this paper, we first focus on the uniqueness of solutions and show for any matrix $Ain mathbb{H}^{m times d}$ there always exists a vector $mathbf{b} in mathbb{R}^m$ such that the solution is not unique. But, in real case, such ``bad'' vectors $mathbf{b}$ are negligible, namely, if $mathbf{b} in mathbb{R}_{+}^m$ does not lie in some measure zero set, then the solution is unique. We also present some conditions under which the solution is unique. For the stability of solutions, we prove that the solution is never uniformly stable. But if we restrict the vectors $mathbf{b}$ to any convex set then it is stable.","PeriodicalId":18456,"journal":{"name":"Mathematics of Computation","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-10-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135302160","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Tamed stability of finite difference schemes for the transport equation on the half-line","authors":"Lucas Coeuret","doi":"10.1090/mcom/3901","DOIUrl":"https://doi.org/10.1090/mcom/3901","url":null,"abstract":"In this paper, we prove that, under precise spectral assumptions, some finite difference approximations of scalar leftgoing transport equations on the positive half-line with numerical boundary conditions are <inline-formula content-type=\"math/mathml\"> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" alttext=\"script l Superscript 1\"> <mml:semantics> <mml:msup> <mml:mi>ℓ<!-- ℓ --></mml:mi> <mml:mn>1</mml:mn> </mml:msup> <mml:annotation encoding=\"application/x-tex\">ell ^1</mml:annotation> </mml:semantics> </mml:math> </inline-formula>-stable but <inline-formula content-type=\"math/mathml\"> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" alttext=\"script l Superscript q\"> <mml:semantics> <mml:msup> <mml:mi>ℓ<!-- ℓ --></mml:mi> <mml:mi>q</mml:mi> </mml:msup> <mml:annotation encoding=\"application/x-tex\">ell ^q</mml:annotation> </mml:semantics> </mml:math> </inline-formula>-unstable for any <inline-formula content-type=\"math/mathml\"> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" alttext=\"q greater-than 1\"> <mml:semantics> <mml:mrow> <mml:mi>q</mml:mi> <mml:mo>></mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:annotation encoding=\"application/x-tex\">q>1</mml:annotation> </mml:semantics> </mml:math> </inline-formula>. The proof relies on the accurate description of the Green’s function for a particular family of finite rank perturbations of Toeplitz operators whose essential spectrum belongs to the closed unit disk and with a simple eigenvalue of modulus <inline-formula content-type=\"math/mathml\"> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" alttext=\"1\"> <mml:semantics> <mml:mn>1</mml:mn> <mml:annotation encoding=\"application/x-tex\">1</mml:annotation> </mml:semantics> </mml:math> </inline-formula> embedded into the essential spectrum.","PeriodicalId":18456,"journal":{"name":"Mathematics of Computation","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135547407","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Minimization of differential equations and algebraic values of 𝐸-functions","authors":"Alin Bostan, Tanguy Rivoal, Bruno Salvy","doi":"10.1090/mcom/3912","DOIUrl":"https://doi.org/10.1090/mcom/3912","url":null,"abstract":"","PeriodicalId":18456,"journal":{"name":"Mathematics of Computation","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135428054","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Numerical analysis for electromagnetic scattering from nonlinear boundary conditions","authors":"Jörg Nick","doi":"10.1090/mcom/3914","DOIUrl":"https://doi.org/10.1090/mcom/3914","url":null,"abstract":"","PeriodicalId":18456,"journal":{"name":"Mathematics of Computation","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135579674","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"A kit for linear forms in three logarithms","authors":"Maurice Mignotte, Paul Voutier, Michel Laurent","doi":"10.1090/mcom/3908","DOIUrl":"https://doi.org/10.1090/mcom/3908","url":null,"abstract":"","PeriodicalId":18456,"journal":{"name":"Mathematics of Computation","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135096432","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}