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.

Palindrome numbers and its reverse are the same

For example, the reverse of 1234 is 4321.

Method

Given a positive integer ${\displaystyle x}$, the integer which has the same digits in the reverse order is:^[1]

${\displaystyle 10^{\lfloor \log _{10}x\rfloor }x-99\sum _{k=1}^{\lfloor \log _{10}x\rfloor }{\lfloor 10^{-k}x\rfloor }10^{\lfloor \log _{10}x\rfloor -k}}$

Applications

Obtaining the reverse of a number is useful to predict if a number is Lychrel or not.^[2][3]