The eigenvalues of the sum of matrices with prescribed invariant polynomials *

Let A and B be n×n matrices over a field F, and c ₁,…,c_n ∈F. We give a sufficient condition for the existence of matrices A' and B' similar to A and B, respectively, such that A' + B' has eigenvalues c ₁,…,c_n .     

A bound for the determinant of the sum of two normal matrices

Hadamard's determinant theorem is used to obtain an upper bound for the modulus of the determinant of the sum of two normal matrices in terms of their eigenvalues. This bound is compared with another given by M. E. Miranda.     

Existence of matrices with prescribed eigenvalues and entries

It is proved improving a previous result of Oliveira that apart from two exceptions there always exists an n×n matrix with arbitarily prescribed 2n-3 entries and spectrum. Moreover, it is shown that the number 2n 3 of prescribed entries cannot be increased.     

The eigenvalues of the product of matrices with prescribed similarity classes *

Let A and B be n × n nonsingular matrices over a field F, and c ₁,…,c _n ∈ F. We give a necessary and sufficient condition for the existence of matrices A′ and B′ similar to A and B, respectively, such that A′ B′ has eigenvalues c ₁,…,c _n.     

On the eigenvalues of matrices with prescribed upper triangular part

Consider a matrix D=[D_i,j] partitioned in submatrices D_i,j where the submatrices D_i,iare square. This paper studies the possible eigenvalues of D, when the blocks D_i,j, with i≤j, are fixed, and the other blocks vary. The result obtained generalizes twotheorems already known.     

Universal adjacency matrices with two eigenvalues

Consider a graph Γ on n vertices with adjacency matrix A and degree sequence (d₁,…,d_n). A universal adjacency matrix of Γ is any matrix in Span {A,D,I,J} with a nonzero coefficient for A, where and I and J are the n×n identity and all-ones matrix, respectively. Thus a universal adjacency matrix is a common generalization of the adjacency, the Laplacian, the signless Laplacian and the Seidel matrix. We investigate graphs for which some universal adjacency matrix has just two eigenvalues. The regular ones are strongly regular, complete or empty, but several other interesting classes occur.     

Computational aspect to the nearest matrix with two prescribed eigenvalues

Given a complex square matrix A and two complex numbers λ₁ and λ₂, we present a method to calculate the distance from A to the set of matrices X that have λ₁ and λ₂ as some of their eigenvalues. We also find the nearest matrix X.     

Products of two involutions with prescribed eigenvalues and some applications

Let F be a field. In [Djokovic, Product of two involutions, Arch. Math. 18 (1967) 582-584] it was proved that a matrix A∈F^n×n can be written as A=BC, for some involutions B,C∈F^n×n, if and only if A is similar to A^-1. In this paper we describe the possible eigenvalues of the matrices B and C.As a consequence, in case charF≠2, we describe the possible similarity classes of (P₁₁⊕P₂₂)P^-1, when the nonsingular matrix P=[P_ij]∈F^n×n, i,j∈{1,2} and P₁₁∈F^s×s, varies.When F is an algebraically closed field and charF≠2, we also describe the possible similarity classes of [A_ij]∈F^n×n, i,j∈{1,2}, when A₁₁ and A₂₂ are square zero matrices and A₁₂ and A₂₁ vary.     

Invariants of linear groups generated by matrices with two nonunit eigenvalues

A theorem on the structure of the algebra of invariants of the commutant of a group generated by pseudoreflections is improved. In particular, it is shown that this algebra is a complete intersection. A series of counterexamples to Stanley's conjecture is constructed in dimension 4. Results supporting this conjecture for primitive groups of large dimension are given.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 114, pp. 120–130, 1982.     

The modified bordering method to evaluate eigenvalues and eigenvectors of normal matrices

A bordering procedure is here proposed to evaluate the eigensystem of hermitian matrices, and more in general of normal matrices, when the spectral decomposition is known of then–1×n–1 principal minor. The procedure is also applicable to special real and nonsymmetric matrices here named quasi-symmetric. The computational cost to write the characteristic polynomial isO(n ²), using a new set of recursive formulas. A modified Brent algorithm is used to find the roots of the polynomial. The eigenvectors are evaluated in a direct way with a computational cost ofO(n ²) for each one. Some numerical considerations indicate where numerical difficulties may occur. Numerical results are given comparing this method with the Givens-Householder one.     

Nonsingularity of the difference and the sum of two idempotent matrices

Groß and Trenkler 1 pointed out that if a difference of idempotent matrices P and Q is nonsingular, then so is their sum, and Koliha et al2 expressed explicitly a condition, which combined with the nonsingularity of P+Q ensures the nonsingularity of P-Q. In the present paper, these results are strengthened by showing that the nonsingularity of P-Q is in fact equivalent to the nonsingularity of any combination aP+bQ-cPQ (where a≠0,b≠0,a+b=c), and the nonsingularity of P+Q is equivalent to the nonsingularity of any combination aP+bQ-cPQ (where a≠0,b≠0,a+b≠c).     

On the eigenvalues of matrices

Summary Let A be an n×n matric with arbitrary complex elements and with eigen-values λ₁, λ₂, ..., λ_n. A method is described for the approximàte determination of max | λ_j | ; characteristical is that prescribing a percentual error the number of elementary operations of the process, necessary to reach such precision, depends only on n and not on the elements of A More general characteristical equations are also considered. To Enrico Bompiani on his scientific Jubilee