Brief Concept 
Euler(17071783) invented a symbolic way to write the statements “ y is function of x” as y=f(x) which is read as “ y is equal to f of x” . The variable x is called independent variable and the variable y is called the dependent variable. 
Definition (function) 
A function f from a set X to a set Y is a rule or a correspondence that assigns to each element x in X a unique element y in Y. The set X is called the domain and set of corresponding elements y in Y is called the range of f. 
Notations: 
Functions are denoted by english alphbets such as f,g,n,H,G and so on.
