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 Schrödinger Operator; Multidimensional Schrödinger Operator; Periodic Schrödinger Operator; Quantum Graphs; Non-linear Schrödinger Equation; References; Index.