This paper is a didactic commentary (a transcription with variations) to the paper of S.R. Foguel { \it Finite Dimensional Perturbations in Banach Spaces}. Addressed, mainly: postgraduates and related readers. Subject: Suppose we have two linear operators, A, B, so that B - A is rank one. Let \lambda_o be an { \it isolated} point of the spectrum of A. In addition, let \lambda_o be an { \it eigenvalue} of A: \lambda_o \in \sigma_{ pp}(A) . The question is: Is \lambda_o an eigenvalue of B ? And, if so, is the multiplicity of \lambda_o in \sigma_{ pp}(B) equal to the multiplicity of \lambda_o in \sigma_{ pp}(A) ? -- or less? -- or greater? Keywords: M.G.Krein's Formula, Finite Rank Perturbation
Categories: math-ph math.DS math.MP