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The maximum multiplicity and the two largest multiplicities of eigenvalues in a Hermitian matrix whose graph is a tree

Rosário Fernandes (2015)

Special Matrices

The maximum multiplicity of an eigenvalue in a matrix whose graph is a tree, M1, was understood fully (froma combinatorial perspective) by C.R. Johnson, A. Leal-Duarte (Linear Algebra and Multilinear Algebra 46 (1999) 139-144). Among the possible multiplicity lists for the eigenvalues of Hermitian matrices whose graph is a tree, we focus upon M2, the maximum value of the sum of the two largest multiplicities when the largest multiplicity is M1. Upper and lower bounds are given for M2. Using a combinatorial...

The Morse landscape of a riemannian disk

S. Frankel, Michael Katz (1993)

Annales de l'institut Fourier

We study upper bounds on the length functional along contractions of loops in Riemannian disks of bounded diameter and circumference. By constructing metrics adapted to imbedded trees of increasing complexity, we reduce the nonexistence of such upper bounds to the study of a topological invariant of imbedded finite trees. This invariant is related to the complexity of the binary representation of integers. It is also related to lower bounds on the number of points in level sets of a real-valued...

The relation between the number of leaves of a tree and its diameter

Pu Qiao, Xingzhi Zhan (2022)

Czechoslovak Mathematical Journal

Let L ( n , d ) denote the minimum possible number of leaves in a tree of order n and diameter d . Lesniak (1975) gave the lower bound B ( n , d ) = 2 ( n - 1 ) / d for L ( n , d ) . When d is even, B ( n , d ) = L ( n , d ) . But when d is odd, B ( n , d ) is smaller than L ( n , d ) in general. For example, B ( 21 , 3 ) = 14 while L ( 21 , 3 ) = 19 . In this note, we determine L ( n , d ) using new ideas. We also consider the converse problem and determine the minimum possible diameter of a tree with given order and number of leaves.

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