Pub Date : 2025-12-16DOI: 10.1016/j.laa.2025.12.013
Aikaterini Aretaki , Maria Adam , Michael Tsatsomeros
It is well known that the eigenvalues of a complex matrix A are located to the left of the vertical line passing through the largest eigenvalue of its Hermitian part, . Adam and Tsatsomeros in [1] defined a cubic algebraic curve, known as the shell of A, using the two largest eigenvalues of . This curve localizes the spectrum further and lies to the left of the aforementioned vertical line. Later, Bergqvist in [5] extended the methodology employed in [1] to define a new curve, , in terms of the three largest eigenvalues of . This article delves into the geometry of for a real matrix A to address some open questions raised in [5]. In particular, specific conditions are established to characterize the configurations of in certain cases. Additionally, the number of eigenvalues of A surrounded by a bounded branch of the curve is examined. Examples are used to validate our findings and demonstrate the quality of as a finer spectrum localization area when compared to .
{"title":"Curves and spectrum localization for real matrices","authors":"Aikaterini Aretaki , Maria Adam , Michael Tsatsomeros","doi":"10.1016/j.laa.2025.12.013","DOIUrl":"10.1016/j.laa.2025.12.013","url":null,"abstract":"<div><div>It is well known that the eigenvalues of a complex matrix <em>A</em> are located to the left of the vertical line passing through the largest eigenvalue of its Hermitian part, <span><math><mi>H</mi><mo>(</mo><mi>A</mi><mo>)</mo></math></span>. Adam and Tsatsomeros in <span><span>[1]</span></span> defined a cubic algebraic curve, known as the <em>shell</em> <span><math><msub><mrow><mi>Γ</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>(</mo><mi>A</mi><mo>)</mo></math></span> of <em>A</em>, using the two largest eigenvalues of <span><math><mi>H</mi><mo>(</mo><mi>A</mi><mo>)</mo></math></span>. This curve localizes the spectrum further and lies to the left of the aforementioned vertical line. Later, Bergqvist in <span><span>[5]</span></span> extended the methodology employed in <span><span>[1]</span></span> to define a new curve, <span><math><msub><mrow><mi>Γ</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>(</mo><mi>A</mi><mo>)</mo></math></span>, in terms of the three largest eigenvalues of <span><math><mi>H</mi><mo>(</mo><mi>A</mi><mo>)</mo></math></span>. This article delves into the geometry of <span><math><msub><mrow><mi>Γ</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>(</mo><mi>A</mi><mo>)</mo></math></span> for a real matrix <em>A</em> to address some open questions raised in <span><span>[5]</span></span>. In particular, specific conditions are established to characterize the configurations of <span><math><msub><mrow><mi>Γ</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>(</mo><mi>A</mi><mo>)</mo></math></span> in certain cases. Additionally, the number of eigenvalues of <em>A</em> surrounded by a bounded branch of the curve is examined. Examples are used to validate our findings and demonstrate the quality of <span><math><msub><mrow><mi>Γ</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>(</mo><mi>A</mi><mo>)</mo></math></span> as a finer spectrum localization area when compared to <span><math><msub><mrow><mi>Γ</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>(</mo><mi>A</mi><mo>)</mo></math></span>.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"733 ","pages":"Pages 116-154"},"PeriodicalIF":1.1,"publicationDate":"2025-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145788853","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}
Pub Date : 2025-12-15DOI: 10.1016/j.laa.2025.12.011
Oksana Bezushchak
D. Benkovič described Jordan homomorphisms of algebras of triangular matrices over a commutative unital ring without additive 2-torsion. We extend this result to the case of noncommutative rings and remove the assumption of additive torsion.
Let R be an associative unital algebra over a commutative unital ring Φ. Consider the algebra of triangular matrices over R, and its subalgebra consisting of matrices whose main diagonal entries lie in Φ. We prove that for any Jordan homomorphism of , its restriction to is standard.
{"title":"Jordan homomorphisms of triangular algebras over noncommutative algebras","authors":"Oksana Bezushchak","doi":"10.1016/j.laa.2025.12.011","DOIUrl":"10.1016/j.laa.2025.12.011","url":null,"abstract":"<div><div>D. Benkovič described Jordan homomorphisms of algebras of triangular matrices over a commutative unital ring without additive 2-torsion. We extend this result to the case of noncommutative rings and remove the assumption of additive torsion.</div><div>Let <em>R</em> be an associative unital algebra over a commutative unital ring Φ. Consider the algebra <span><math><msub><mrow><mi>T</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>(</mo><mi>R</mi><mo>)</mo></math></span> of triangular <span><math><mi>n</mi><mo>×</mo><mi>n</mi></math></span> matrices over <em>R</em>, and its subalgebra <span><math><msubsup><mrow><mi>T</mi></mrow><mrow><mi>n</mi></mrow><mrow><mn>0</mn></mrow></msubsup><mo>(</mo><mi>R</mi><mo>)</mo></math></span> consisting of matrices whose main diagonal entries lie in Φ. We prove that for any Jordan homomorphism of <span><math><msub><mrow><mi>T</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>(</mo><mi>R</mi><mo>)</mo></math></span>, its restriction to <span><math><msubsup><mrow><mi>T</mi></mrow><mrow><mi>n</mi></mrow><mrow><mn>0</mn></mrow></msubsup><mo>(</mo><mi>R</mi><mo>)</mo></math></span> is standard.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"733 ","pages":"Pages 61-74"},"PeriodicalIF":1.1,"publicationDate":"2025-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145788851","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}
Pub Date : 2025-12-15DOI: 10.1016/j.laa.2025.12.007
Vance Faber , Jörg Liesen , Petr Tichý
We derive, similar to Lau and Riha in [22], a matrix formulation of a general best approximation theorem of Singer for the special case of spectral approximations of a given matrix from a given subspace. Using our matrix formulation we describe the relation of the spectral approximation problem to semidefinite programming, and we present a simple MATLAB code to solve the problem numerically. We then obtain geometric characterizations of spectral approximations that are based on the k-dimensional field of k matrices, which we illustrate with several numerical examples. The general spectral approximation problem is a min-max problem, whose value is bounded from below by the corresponding max-min problem. Using our geometric characterizations of spectral approximations, we derive several necessary and sufficient as well as sufficient conditions for equality of the max-min and min-max values. Finally, we prove that the max-min and min-max values are always equal for block diagonal matrices containing two identical diagonal blocks. Several results in this paper generalize results that have been obtained in the convergence analysis of the GMRES method for solving linear algebraic systems.
{"title":"Matrix best approximation in the spectral norm","authors":"Vance Faber , Jörg Liesen , Petr Tichý","doi":"10.1016/j.laa.2025.12.007","DOIUrl":"10.1016/j.laa.2025.12.007","url":null,"abstract":"<div><div>We derive, similar to Lau and Riha in <span><span>[22]</span></span>, a matrix formulation of a general best approximation theorem of Singer for the special case of spectral approximations of a given matrix from a given subspace. Using our matrix formulation we describe the relation of the spectral approximation problem to semidefinite programming, and we present a simple MATLAB code to solve the problem numerically. We then obtain geometric characterizations of spectral approximations that are based on the <em>k</em>-dimensional field of <em>k</em> matrices, which we illustrate with several numerical examples. The general spectral approximation problem is a min-max problem, whose value is bounded from below by the corresponding max-min problem. Using our geometric characterizations of spectral approximations, we derive several necessary and sufficient as well as sufficient conditions for equality of the max-min and min-max values. Finally, we prove that the max-min and min-max values are always equal for <span><math><mn>2</mn><mo>×</mo><mn>2</mn></math></span> block diagonal matrices containing two identical diagonal blocks. Several results in this paper generalize results that have been obtained in the convergence analysis of the GMRES method for solving linear algebraic systems.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"733 ","pages":"Pages 178-204"},"PeriodicalIF":1.1,"publicationDate":"2025-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145880819","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}
Pub Date : 2025-12-15DOI: 10.1016/j.laa.2025.12.010
John Byrne
For a graph family , let and denote the maximum number of edges and maximum spectral radius of an n-vertex -free graph, respectively, and let and denote the corresponding sets of extremal graphs. Wang, Kang, and Xue showed that if and then for n large enough. Fang, Tait, and Zhai extended this result by showing if then for n large enough, and asked for the maximum constant such that guarantees such containment. In this paper we determine exactly for all .
{"title":"A sharp spectral extremal result for general non-bipartite graphs","authors":"John Byrne","doi":"10.1016/j.laa.2025.12.010","DOIUrl":"10.1016/j.laa.2025.12.010","url":null,"abstract":"<div><div>For a graph family <span><math><mi>F</mi></math></span>, let <span><math><mrow><mi>ex</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo></math></span> and <span><math><mrow><mi>spex</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo></math></span> denote the maximum number of edges and maximum spectral radius of an <em>n</em>-vertex <span><math><mi>F</mi></math></span>-free graph, respectively, and let <span><math><mrow><mi>EX</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo></math></span> and <span><math><mrow><mi>SPEX</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo></math></span> denote the corresponding sets of extremal graphs. Wang, Kang, and Xue showed that if <span><math><mi>r</mi><mo>≥</mo><mn>2</mn></math></span> and <span><math><mrow><mi>ex</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo><mo>=</mo><mi>e</mi><mo>(</mo><msub><mrow><mi>T</mi></mrow><mrow><mi>n</mi><mo>,</mo><mi>r</mi></mrow></msub><mo>)</mo><mo>+</mo><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math></span> then <span><math><mrow><mi>SPEX</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo><mo>⊆</mo><mrow><mi>EX</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo></math></span> for <em>n</em> large enough. Fang, Tait, and Zhai extended this result by showing if <span><math><mi>e</mi><mo>(</mo><msub><mrow><mi>T</mi></mrow><mrow><mi>n</mi><mo>,</mo><mi>r</mi></mrow></msub><mo>)</mo><mo>≤</mo><mrow><mi>ex</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo><mo><</mo><mi>e</mi><mo>(</mo><msub><mrow><mi>T</mi></mrow><mrow><mi>n</mi><mo>,</mo><mi>r</mi></mrow></msub><mo>)</mo><mo>+</mo><mo>⌊</mo><mi>n</mi><mo>/</mo><mn>2</mn><mi>r</mi><mo>⌋</mo></math></span> then <span><math><mrow><mi>SPEX</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo><mo>⊆</mo><mrow><mi>EX</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo></math></span> for <em>n</em> large enough, and asked for the maximum constant <span><math><mi>c</mi><mo>(</mo><mi>r</mi><mo>)</mo></math></span> such that <span><math><mrow><mi>ex</mi></mrow><mo>(</mo><mi>n</mi><mo>,</mo><mi>F</mi><mo>)</mo><mo>≤</mo><mi>e</mi><mo>(</mo><msub><mrow><mi>T</mi></mrow><mrow><mi>n</mi><mo>,</mo><mi>r</mi></mrow></msub><mo>)</mo><mo>+</mo><mo>(</mo><mi>c</mi><mo>(</mo><mi>r</mi><mo>)</mo><mo>−</mo><mi>ε</mi><mo>)</mo><mi>n</mi></math></span> guarantees such containment. In this paper we determine <span><math><mi>c</mi><mo>(</mo><mi>r</mi><mo>)</mo></math></span> exactly for all <span><math><mi>r</mi><mo>≥</mo><mn>3</mn></math></span>.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"733 ","pages":"Pages 75-115"},"PeriodicalIF":1.1,"publicationDate":"2025-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145788852","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}
Pub Date : 2025-12-12DOI: 10.1016/j.laa.2025.12.008
Filip Jonsson Kling
Consider a standard graded artinian k-algebra B and an extension of B by a new variable, for some . We will show how maximal rank properties for powers of a general linear form on A can be determined by maximal rank properties for different powers of general linear forms on B. This is then used to study Lefschetz properties of algebras that can be obtained via such extensions. In particular, it allows for a new proof that monomial complete intersections have the strong Lefschetz property over a field of characteristic zero. Moreover, it gives a recursive formula for the determinants that show up in that case. Finally, for algebras over a field of characteristic zero, we give a classification for what properties B must have for all extensions to have the weak or the strong Lefschetz property.
{"title":"Preserving Lefschetz properties after extension of variables","authors":"Filip Jonsson Kling","doi":"10.1016/j.laa.2025.12.008","DOIUrl":"10.1016/j.laa.2025.12.008","url":null,"abstract":"<div><div>Consider a standard graded artinian <em>k</em>-algebra <em>B</em> and an extension of <em>B</em> by a new variable, <span><math><mi>A</mi><mo>=</mo><mi>B</mi><msub><mrow><mo>⊗</mo></mrow><mrow><mi>k</mi></mrow></msub><mi>k</mi><mo>[</mo><mi>x</mi><mo>]</mo><mo>/</mo><mo>(</mo><msup><mrow><mi>x</mi></mrow><mrow><mi>d</mi></mrow></msup><mo>)</mo></math></span> for some <span><math><mi>d</mi><mo>≥</mo><mn>1</mn></math></span>. We will show how maximal rank properties for powers of a general linear form on <em>A</em> can be determined by maximal rank properties for different powers of general linear forms on <em>B</em>. This is then used to study Lefschetz properties of algebras that can be obtained via such extensions. In particular, it allows for a new proof that monomial complete intersections have the strong Lefschetz property over a field of characteristic zero. Moreover, it gives a recursive formula for the determinants that show up in that case. Finally, for algebras over a field of characteristic zero, we give a classification for what properties <em>B</em> must have for all extensions <span><math><mi>B</mi><msub><mrow><mo>⊗</mo></mrow><mrow><mi>k</mi></mrow></msub><mi>k</mi><mo>[</mo><mi>x</mi><mo>]</mo><mo>/</mo><mo>(</mo><msup><mrow><mi>x</mi></mrow><mrow><mi>d</mi></mrow></msup><mo>)</mo></math></span> to have the weak or the strong Lefschetz property.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"733 ","pages":"Pages 26-60"},"PeriodicalIF":1.1,"publicationDate":"2025-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145788850","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}
Pub Date : 2025-12-08DOI: 10.1016/j.laa.2025.12.005
Hwa-Long Gau , Chi-Kwong Li , Kuo-Zhong Wang
For an complex matrix A, we study the value , which is the maximum size of an orthonormal set such that lie on the boundary of for . We give a complete characterization of matrices A with , and determine when such a matrix has reducing subspaces. Furthermore, we characterize companion matrices and nonnegative upper triangular the Toeplitz matrices A with .
{"title":"Matrices with all diagonal entries lying on the boundary of the numerical range","authors":"Hwa-Long Gau , Chi-Kwong Li , Kuo-Zhong Wang","doi":"10.1016/j.laa.2025.12.005","DOIUrl":"10.1016/j.laa.2025.12.005","url":null,"abstract":"<div><div>For an <span><math><mi>n</mi><mo>×</mo><mi>n</mi></math></span> complex matrix <em>A</em>, we study the value <span><math><mi>k</mi><mo>(</mo><mi>A</mi><mo>)</mo></math></span>, which is the maximum size of an orthonormal set <span><math><mo>{</mo><msub><mrow><mi>x</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>,</mo><mo>…</mo><mo>,</mo><msub><mrow><mi>x</mi></mrow><mrow><mi>k</mi></mrow></msub><mo>}</mo></math></span> such that <span><math><msubsup><mrow><mi>x</mi></mrow><mrow><mi>j</mi></mrow><mrow><mo>⁎</mo></mrow></msubsup><mi>A</mi><msub><mrow><mi>x</mi></mrow><mrow><mi>j</mi></mrow></msub></math></span> lie on the boundary of <span><math><mi>W</mi><mo>(</mo><mi>A</mi><mo>)</mo></math></span> for <span><math><mi>j</mi><mo>=</mo><mn>1</mn><mo>,</mo><mo>…</mo><mo>,</mo><mi>k</mi></math></span>. We give a complete characterization of matrices <em>A</em> with <span><math><mi>k</mi><mo>(</mo><mi>A</mi><mo>)</mo><mo>=</mo><mi>n</mi></math></span>, and determine when such a matrix has reducing subspaces. Furthermore, we characterize companion matrices and nonnegative upper triangular the Toeplitz matrices <em>A</em> with <span><math><mi>k</mi><mo>(</mo><mi>A</mi><mo>)</mo><mo>=</mo><mi>n</mi></math></span>.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"733 ","pages":"Pages 1-25"},"PeriodicalIF":1.1,"publicationDate":"2025-12-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145711964","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}
Pub Date : 2025-12-05DOI: 10.1016/j.laa.2025.12.004
Mao-Ting Chien , Hiroshi Nakazato
The ternary form of an matrix A is defined by , where and . If the algebraic curve has no singular points, the Helton-Vinnikov theorem asserts that there are non-unitarily similar symmetric matrices S satisfying , where . We compare the operator norms of the symmetric matrices that share the same numerical range of A.
{"title":"Comparing the operator norms of symmetric matrices sharing the same numerical range","authors":"Mao-Ting Chien , Hiroshi Nakazato","doi":"10.1016/j.laa.2025.12.004","DOIUrl":"10.1016/j.laa.2025.12.004","url":null,"abstract":"<div><div>The ternary form of an <span><math><mi>n</mi><mo>×</mo><mi>n</mi></math></span> matrix <em>A</em> is defined by <span><math><msub><mrow><mi>F</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mi>t</mi><mo>,</mo><mi>x</mi><mo>,</mo><mi>y</mi><mo>)</mo><mo>=</mo><mrow><mi>det</mi></mrow><mo>(</mo><mi>t</mi><msub><mrow><mi>I</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>+</mo><mi>x</mi><mo>ℜ</mo><mo>(</mo><mi>A</mi><mo>)</mo><mo>+</mo><mi>y</mi><mo>ℑ</mo><mo>(</mo><mi>A</mi><mo>)</mo><mo>)</mo></math></span>, where <span><math><mo>ℜ</mo><mo>(</mo><mi>A</mi><mo>)</mo><mo>=</mo><mo>(</mo><mi>A</mi><mo>+</mo><msup><mrow><mi>A</mi></mrow><mrow><mo>⁎</mo></mrow></msup><mo>)</mo><mo>/</mo><mn>2</mn></math></span> and <span><math><mo>ℑ</mo><mo>(</mo><mi>A</mi><mo>)</mo><mo>=</mo><mo>(</mo><mi>A</mi><mo>−</mo><msup><mrow><mi>A</mi></mrow><mrow><mo>⁎</mo></mrow></msup><mo>)</mo><mo>/</mo><mo>(</mo><mn>2</mn><mi>i</mi><mo>)</mo></math></span>. If the algebraic curve <span><math><msub><mrow><mi>F</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mi>t</mi><mo>,</mo><mi>x</mi><mo>,</mo><mi>y</mi><mo>)</mo><mo>=</mo><mn>0</mn></math></span> has no singular points, the Helton-Vinnikov theorem asserts that there are <span><math><msup><mrow><mn>2</mn></mrow><mrow><mi>g</mi></mrow></msup></math></span> non-unitarily similar symmetric matrices <em>S</em> satisfying <span><math><msub><mrow><mi>F</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mi>t</mi><mo>,</mo><mi>x</mi><mo>,</mo><mi>y</mi><mo>)</mo><mo>=</mo><msub><mrow><mi>F</mi></mrow><mrow><mi>S</mi></mrow></msub><mo>(</mo><mi>t</mi><mo>,</mo><mi>x</mi><mo>,</mo><mi>y</mi><mo>)</mo></math></span>, where <span><math><mi>g</mi><mo>=</mo><mo>(</mo><mi>n</mi><mo>−</mo><mn>1</mn><mo>)</mo><mo>(</mo><mi>n</mi><mo>−</mo><mn>2</mn><mo>)</mo><mo>/</mo><mn>2</mn></math></span>. We compare the operator norms of the <span><math><msup><mrow><mn>2</mn></mrow><mrow><mi>g</mi></mrow></msup></math></span> symmetric matrices that share the same numerical range of <em>A</em>.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"732 ","pages":"Pages 207-228"},"PeriodicalIF":1.1,"publicationDate":"2025-12-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145733919","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}
Pub Date : 2025-12-04DOI: 10.1016/j.laa.2025.12.002
Krzysztof Szczygielski
We characterize covariant positive decomposable maps between unital C*-algebras in terms of a dilation theorem, which generalizes a seminal result by H. Scutaru from (1979) [7]. As a case study, we provide a certain characterization of the operator sum representation of maps on , covariant with respect to the maximal commutative subgroup of . A connection to quantum dynamics is established by specifying sufficient and necessary conditions for covariance of D-divisible (decomposably divisible) quantum evolution families, recently introduced in Szczygielski (2023) [11].
{"title":"Covariant decomposable maps on C*-algebras and quantum dynamics","authors":"Krzysztof Szczygielski","doi":"10.1016/j.laa.2025.12.002","DOIUrl":"10.1016/j.laa.2025.12.002","url":null,"abstract":"<div><div>We characterize covariant positive decomposable maps between unital C*-algebras in terms of a dilation theorem, which generalizes a seminal result by H. Scutaru from (1979) <span><span>[7]</span></span>. As a case study, we provide a certain characterization of the operator sum representation of maps on <span><math><msub><mrow><mi>M</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>(</mo><mi>C</mi><mo>)</mo></math></span>, covariant with respect to the maximal commutative subgroup of <span><math><mi>U</mi><mo>(</mo><mi>n</mi><mo>)</mo></math></span>. A connection to quantum dynamics is established by specifying sufficient and necessary conditions for covariance of D-divisible (decomposably divisible) quantum evolution families, recently introduced in Szczygielski (2023) <span><span>[11]</span></span>.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"732 ","pages":"Pages 126-161"},"PeriodicalIF":1.1,"publicationDate":"2025-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145682616","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}
Pub Date : 2025-12-03DOI: 10.1016/j.laa.2025.11.023
Grigory Ivanov
We prove the following colorful Helly-type result: Fix . Assume are finite sets (colors) of nonzero vectors in . If for every rainbow sub-selection R from these sets of size at most , the system has at least k linearly independent solutions, then at least one of the systems , has at least k linearly independent solutions.
A rainbow sub-selection from several sets refers to choosing at most one element from each set (color).
The Helly number and the number of colors are optimal.
Our key observation is a certain colorful Carathéodory-type result for positive bases.
{"title":"Colorful positive bases decomposition and Helly-type results for cones","authors":"Grigory Ivanov","doi":"10.1016/j.laa.2025.11.023","DOIUrl":"10.1016/j.laa.2025.11.023","url":null,"abstract":"<div><div>We prove the following colorful Helly-type result: Fix <span><math><mi>k</mi><mo>∈</mo><mo>[</mo><mi>d</mi><mo>−</mo><mn>1</mn><mo>]</mo></math></span>. Assume <span><math><msub><mrow><mi>A</mi></mrow><mrow><mn>1</mn></mrow></msub><mo>,</mo><mo>…</mo><mo>,</mo><msub><mrow><mi>A</mi></mrow><mrow><mi>d</mi><mo>+</mo><mo>(</mo><mi>d</mi><mo>−</mo><mi>k</mi><mo>)</mo><mo>+</mo><mn>1</mn></mrow></msub></math></span> are finite sets (colors) of nonzero vectors in <span><math><msup><mrow><mi>R</mi></mrow><mrow><mi>d</mi></mrow></msup></math></span>. If for every rainbow sub-selection <em>R</em> from these sets of size at most <span><math><mi>max</mi><mo></mo><mo>{</mo><mi>d</mi><mo>+</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>(</mo><mi>d</mi><mo>−</mo><mi>k</mi><mo>+</mo><mn>1</mn><mo>)</mo><mo>}</mo></math></span>, the system <span><math><mrow><mo>〈</mo><mi>a</mi><mo>,</mo><mi>x</mi><mo>〉</mo></mrow><mo>≤</mo><mn>0</mn><mo>,</mo><mspace></mspace><mi>a</mi><mo>∈</mo><mi>R</mi></math></span> has at least <em>k</em> linearly independent solutions, then at least one of the systems <span><math><mrow><mo>〈</mo><mi>a</mi><mo>,</mo><mi>x</mi><mo>〉</mo></mrow><mo>≤</mo><mn>0</mn><mo>,</mo><mspace></mspace><mi>a</mi><mo>∈</mo><msub><mrow><mi>A</mi></mrow><mrow><mi>i</mi></mrow></msub></math></span>, <span><math><mi>i</mi><mo>∈</mo><mo>[</mo><mi>d</mi><mo>+</mo><mo>(</mo><mi>d</mi><mo>−</mo><mi>k</mi><mo>)</mo><mo>+</mo><mn>1</mn><mo>]</mo></math></span> has at least <em>k</em> linearly independent solutions.</div><div>A <em>rainbow sub-selection</em> from several sets refers to choosing at most one element from each set (color).</div><div>The Helly number <span><math><mi>max</mi><mo></mo><mo>{</mo><mi>d</mi><mo>+</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>(</mo><mi>d</mi><mo>−</mo><mi>k</mi><mo>+</mo><mn>1</mn><mo>)</mo><mo>}</mo></math></span> and the number of colors <span><math><mi>d</mi><mo>+</mo><mo>(</mo><mi>d</mi><mo>−</mo><mi>k</mi><mo>)</mo><mo>+</mo><mn>1</mn></math></span> are optimal.</div><div>Our key observation is a certain colorful Carathéodory-type result for positive bases.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"732 ","pages":"Pages 108-125"},"PeriodicalIF":1.1,"publicationDate":"2025-12-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145682613","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}
Pub Date : 2025-12-03DOI: 10.1016/j.laa.2025.12.001
Michał Buchała
The aim of this paper is to obtain m-isometric dilation of expansive m-concave operator on Hilbert space. The obtained dilation is shown to be minimal. The matrix representation of this dilation is given. It is also proved that in case of 3-concave operators the assumption on expansivity is not necessary. The paper contains an example showing that minimal m-isometric dilations may not be isomorphic.
{"title":"Every expansive m-concave operator has m-isometric dilation","authors":"Michał Buchała","doi":"10.1016/j.laa.2025.12.001","DOIUrl":"10.1016/j.laa.2025.12.001","url":null,"abstract":"<div><div>The aim of this paper is to obtain <em>m</em>-isometric dilation of expansive <em>m</em>-concave operator on Hilbert space. The obtained dilation is shown to be minimal. The matrix representation of this dilation is given. It is also proved that in case of 3-concave operators the assumption on expansivity is not necessary. The paper contains an example showing that minimal <em>m</em>-isometric dilations may not be isomorphic.</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"732 ","pages":"Pages 93-107"},"PeriodicalIF":1.1,"publicationDate":"2025-12-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145682612","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}