CONTENTS: Preface; A Bit of Quantum Mechanics; Operators in Hilbert Spaces; Spectral Theorem for Self-adjoint Operators; Compact Operators and the Hilbert-Schmidt Theorem; Elements of Perturbation Theory; Variational Principles; One-Dimensional Schrodinger Operator; Multidimensional Schrodinger Operator; Periodic Schrodinger Operator; Quantum Graphs; Non-linear Schrodinger Equation; References; Index.