  This paper is a didactic comment (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,
  T_1, T_2, so that
  T_2^{-1} - T_1^{-1} is rank-one.
  What we want to know is how we can express
  (z-T_2)^{-1} in terms of
  T_2^{-1} - T_1^{-1} and
  (z-T_1)^{-1}
  .
  Keywords: M.G.Krein's Formula, Finite Rank Perturbations.
