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.

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