Home > ELA > Vol. 30 (2015)

#### Keywords

Tree, Eigenvalues, Hermitian matrices, Multiplicity, Parter vertex

#### Abstract

For an Hermitian matrix whose graph is a tree and for a given eigenvalue having Parter vertices, the possibilities for the multiplicity are considered. If V = {v_1, . . . , v_k} is a fragmenting Parter set in a tree relative to the eigenvalue , and T_{i+1} is the component of T−{v_1, v_2, . . . , v_i} in which v_{i+1} lies, it is shown that \sum_{i}^K N_i=m_A(\lambda)+2k−1, in which N_i is the number of components of T_i−v_i in which lambda is an eigenvalue. This identity is applied to make several observations, including about when a set of strong Parter vertices leaves only 3 components with \lambda and about multiplicities in binary trees. Furthermore, it is shown that one can construct an Hermitian matrix whose graph is a tree that has a strong Parter set V such that |V | = k for each k in 1 <= k<= m − 1 for given multiplicity m >= 2 of an eigenvalue lambda. Finally, some examples are given, in which the notion of afragmenting Parter set is used.

#### Recommended Citation

Toyonaga, Kenji and Johnson, Charles R..
(2015),
"Application of an identity for subtrees with a given eigenvalue",
*Electronic Journal of Linear Algebra*,
Volume 30, pp. 964-973.

DOI: http://dx.doi.org/10.13001/1081-3810.3215

*Abstract*