CONTENTS Introduction Chapter 1. The perturbation of symmetric and non-symmetric matrices § 1. Introduction § 2. The perturbation of solutions of linear algebraic equations § 3. The perturbation of eigenvalues and eigenvectors Chapter 2. The perturbation of selfadjoint and non-selfadjoint boundary problems § 4. Perturbation without raising the order of the differential operator § 5. Perturbation by operators of higher order § 6. The case in which the limit problem is on the spectrum § 7. The perturbation of eigenvalues and eigenfunctions Appendix I. The perturbation of eigenvalues and eigenvectors (the general case) Appendix II. The convergence of the series for λε and vε Appendix III. The perturbation of boundary problems by means of differential operators of higher order (general boundary conditions) References