No Arabic abstract
We present a survey of some recent results concerning the location and the Weyl formula for the complex eigenvalues of two non self-adjoint operators. We study the eigenvalues of the generator $G$ of the contraction semigroup $e^{tG}, : t geq 0,$ related to the wave equation in an unbounded domain $Omega$ with dissipative boundary conditions on $partial Omega$. Also one examines the interior transmission eigenvalues (ITE) in a bounded domain $K$ obtaining a Weyl formula with remainder for the counting function $N(r)$ of complex (ITE). The analysis is based on a semi-classical approach.
We prove the absence of eigenvaues of the three-dimensional Dirac operator with non-Hermitian potentials in unbounded regions of the complex plane under smallness conditions on the potentials in Lebesgue spaces. Our sufficient conditions are quantitative and easily checkable.
This note aims to give prominence to some new results on the absence and localization of eigenvalues for the Dirac and Klein-Gordon operators, starting from known resolvent estimates already established in the literature combined with the renowned Birman-Schwinger principle.
We extend the classical boundary values begin{align*} & g(a) = - W(u_{a}(lambda_0,.), g)(a) = lim_{x downarrow a} frac{g(x)}{hat u_{a}(lambda_0,x)}, &g^{[1]}(a) = (p g)(a) = W(hat u_{a}(lambda_0,.), g)(a) = lim_{x downarrow a} frac{g(x) - g(a) hat u_{a}(lambda_0,x)}{u_{a}(lambda_0,x)} end{align*} for regular Sturm-Liouville operators associated with differential expressions of the type $tau = r(x)^{-1}[-(d/dx)p(x)(d/dx) + q(x)]$ for a.e. $xin[a,b] subset mathbb{R}$, to the case where $tau$ is singular on $(a,b) subseteq mathbb{R}$ and the associated minimal operator $T_{min}$ is bounded from below. Here $u_a(lambda_0, cdot)$ and $hat u_a(lambda_0, cdot)$ denote suitably normalized principal and nonprincipal solutions of $tau u = lambda_0 u$ for appropriate $lambda_0 in mathbb{R}$, respectively. We briefly discuss the singular Weyl-Titchmarsh-Kodaira $m$-function and finally illustrate the theory in some detail with the examples of the Bessel, Legendre, and Kummer (resp., Laguerre) operators.
In this paper we study a family of operators dependent on a small parameter $epsilon > 0$, which arise in a problem in fluid mechanics. We show that the spectra of these operators converge to N as $epsilon to 0$, even though, for fixed $epsilon > 0$, the eigenvalue asymptotics are quadratic.
We prove that the eigenvalues of a certain highly non-self-adjoint operator that arises in fluid mechanics correspond, up to scaling by a positive constant, to those of a self-adjoint operator with compact resolvent; hence there are infinitely many real eigenvalues which accumulate only at $pm infty$. We use this result to determine the asymptotic distribution of the eigenvalues and to compute some of the eigenvalues numerically. We compare these to earlier calculations by other authors.