1. Introduction 1.1. Preface 1.2. An Illustrative Perturbed Problem 1.3. A Brief Summary of the Book 2. Theoretical Background 2.1. Linear Functional Analysis 2.2. Nonlinear Functional Analysis 2.2.1. Implicit Function Theorem 2.2.2. Lyapunov-Schmidt Method 2.2.3. Leray-Schauder Degree 2.3. Differential Topology 2.3.1. Differentiable Manifolds 2.3.2. Symplectic Surfaces 2.3.3. Intersection Numbers of Manifolds 2.3.4. Brouwer Degree on Manifolds 2.3.5...