Spectral points of positive and negative type, and type π+ and type π− for closed linear operators and relations in Krein spaces are introduced with the help of approximative eigensequences. The main objective of the paper is to study these sign type properties in the non-selfadjoint case under various kinds of perturbations, e.g. compact perturbations and perturbations small in the gap metric....

Oscillatory matrices were introduced in the seminal work of Gantmacher and Krein. An n × matrix A is called oscillatory if all its minors are nonnegative there exists a positive integer k such that positive. The smallest for which this holds exponent . Krein showed always smaller than or equal to − 1 important nontrivial problem determine exact value exponent. Here we use successive elementary ...

Let J be an involutive Hermitian matrix with signature (t, n− t), 0 ≤ t ≤ n, that is, with t positive and n− t negative eigenvalues. The Krein space numerical range of a complex matrix A of size n is the collection of complex numbers of the form ξ ∗JAξ ξ∗Jξ , with ξ ∈ Cn and ξ∗Jξ = 0. In this note, a class of tridiagonal matrices with hyperbolical numerical range is investigated. A Matlab progr...

According to classical results by M. G. Krein and L. de Branges, for every positive measure μ on the real line R such that ∫ R dμ(t) 1+t2 <∞ there exists a Hamiltonian H such that μ is the spectral measure for the corresponding canonical Hamiltonian system JX ′ = zHX. In the case where μ is an even measure from Steklov class on R, we show that the Hamiltonian H normalized by detH = 1 belongs to...

A b s t r a c t . Let ~A,B be the Krein spectral shift function for a pair of operators A, B, with C = A B trace class. We establish the bound f F(I~A,B()~)I ) d,~ <_ f F ( 1 5 1 c l , o ( ) , ) l ) d A = ~ [F(j) F ( j 1 ) ] # j ( C ) , j= l where F is any non-negative convex function on [0, oo) with F(O) = 0 and #j (C) are the singular values of C. The choice F(t) = t p, p > 1, improves a rece...

We provide a new proof of a theorem of Birman and Solomyak that if A(s) = A0+ sB with B ≥ 0 trace class and dμs(·) = Tr(BEA(s)(·)B), then ∫ 1 0 [dμs(λ)] ds = ξ(λ)dλ where ξ is the Krein spectral shift from A(0) to A(1). Our main point is that this is a simple consequence of the formula: d ds Tr(f(A(s)) = Tr(Bf ′(A(s))). Let A and C = A+B be bounded self-adjoint operators and suppose that B ≥ 0 ...

The aim of this note is to provide the complete characterization of the numerical range of linear operators on the 2-dimensional Krein space C.