Reverse of a number

The reverse of a number is a mathematical method to obtain from a number another written in the opposite way to the first.
For example, the reverse of 1234 is 4321. 
Method.
Given a positive integer formula_1, the integer which has the same digits in the reverse order is:
formula_2
Applications.
Obtaining the reverse of a number is useful to predict if a number is Lychrel or not.