A characterization of the eigenvalues of a completely continuous selfadjoint operator Joachim Naumann (1972) Commentationes Mathematicae Universitatis Carolinae
A remark on the d-characteristic and the d Ξ -characteristic of linear operators in a Banach space Štefan Schwabik (1973) Studia Mathematica
Accurate asymptotic formulas for eigenvalues and eigenfunctions of a boundary-value problem of fourth order. Menken, Hamza (2010) Boundary Value Problems [electronic only]
Automatic continuity of operational calculi on algebras of differentiable functions Neumann, Michael (1980) Abstracta. 8th Winter School on Abstract Analysis