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    Please use this identifier to cite or link to this item: http://tkuir.lib.tku.edu.tw:8080/dspace/handle/987654321/41389

    Title: On matrices whose numerical ranges have circular or weak circular symmetry
    Authors: 譚必信;Tam, Bit-shun;Yang, Shangjun
    Contributors: 淡江大學數學學系
    Keywords: Nonnegative matrix;Numerical range;Numerical radius;Circular disk;Convex polygon;Sharp point;Cyclic index;Diagonal similarity;Cycle;Cyclically m-partite digraph;Linearly partite digraph;Ray pattern
    Date: 1999-12-01
    Issue Date: 2010-01-28 07:27:19 (UTC+8)
    Publisher: Elsevier
    Abstract: In [18] among other equivalent conditions, it is proved that a square complex matrix A is permutationally similar to a block-shift matrix if and only if for any complex matrix B with the same zero pattern as A, W(B), the numerical range of B, is a circular disk centered at the origin. In this paper, we add a long list of further new equivalent conditions. The corresponding result for the numerical range of a square complex matrix to be invariant under a rotation about the origin through an angle of 2π/m, where m⩾2 is a given positive integer, is also proved. Many interesting by-products are obtained. In particular, on the numerical range of a square nonnegative matrix A, the following unexpected results are established: (i) when the undirected graph of A is connected, if W(A) is a circular disk centered at the origin, then so is W(B), for any complex matrix B with the same zero pattern as A; (ii) when A is irreducible, if λ is an eigenvalue in the peripheral spectrum of A that lies on the boundary of W(A), then λ is a sharp point of W(A). We also obtain results on the numerical range of an irreducible square nonnegative matrix, which strengthen or clarify the work of Issos [9] and Nylen and Tam [14] on this topic. Open questions are posed at the end.
    Relation: Linear Algebra and Its Applications 302-303, pp.193-221
    DOI: 10.1016/S0024-3795(99)00174-3
    Appears in Collections:[數學學系暨研究所] 期刊論文

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