Approximately Invariant Subspaces for Unbounded Linear Operators. II. Palle E.T. Jorgensen (1977) Mathematische Annalen
Approximation with Restricted Spectra. C.K. Chui, P.W. Smith, J.D. Ward (1975) Mathematische Zeitschrift
Asymptotic Behaviour of the Solution of a Semilinear Parabolic Equation. Laurent Veron, Abdelilah Gmira (1982) Monatshefte für Mathematik