References:
- Inder J. Taneja, Single Digits Representations of Numbers from 1 to 20000, https://inderjtaneja.com/2019/01/01/single-letter-representations-of-numbers-from-1-to-20000/
- Inder J. Taneja, Single Letter Representations Of Natural Numbers, https://inderjtaneja.com/2017/08/19/single-letter-representations-of-natural-numbers/