The structure of matrices with a maximum multiplicity eigenvalue |
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Authors: | Charles R Johnson |
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Institution: | a Department of Mathematics, College of William and Mary, P.O. Box 8795, Williamsburg, VA 23187-8795, USA b CMUC, Department of Mathematics, University of Coimbra, Apartado 3008, 3001-454 Coimbra, Portugal c Departamento de Matemática, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, 2829-516 Quinta da Torre, Portugal |
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Abstract: | There is remarkable and distinctive structure among Hermitian matrices, whose graph is a given tree T and that have an eigenvalue of multiplicity that is a maximum for T. Among such structure, we give several new results: (1) no vertex of T may be “neutral”; (2) neutral vertices may occur if the largest multiplicity is less than the maximum; (3) every Parter vertex has at least two downer branches; (4) removal of a Parter vertex changes the status of no other vertex; and (5) every set of Parter vertices forms a Parter set. Statements (3), (4) and (5) are also not generally true when the multiplicity is less than the maximum. Some of our results are used to give further insights into prior results, and both the review of necessary background and the development of new structural lemmas may be of independent interest. |
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Keywords: | Hermitian matrices Eigenvalues Multiplicities Maximum multiplicity Trees Path cover number Parter vertices |
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