1. Introduction

2. Preliminary Definitions  
2.1. $p$-variation, Jordan variation, Riesz variation  
2.2. Baire functions

3. Two types of non-linear operators: (left) composition operators and superposition operators  
3.1. Composition Operators  
3.2. Superposition Operators

4. A type of linear operators: right composition operators

5. Acknowledgment