O.O. Ibrogimov, H. Langer, M. Langer and C. Tretter,
Essential spectrum of systems of singular differential equations,
Acta Sci. Math. (Szeged) 79 (2013), 423–465
Abstract:
In this paper we develop a new method to determine the essential spectrum
of coupled systems of singular differential equations.
Applications to problems from magnetohydrodynamics and astrophysics are given.