666 is a natural number after 665 and before 667. It is a number with many properties. One of these properties is that it is the sum of the first 36 natural numbers. That makes 666 a triangular number. 666 is the sum of the first 144 decimals (digits after the decimal point) of pi. The bible says that the number 666 is associated with the devil.
- Alfred S. Posamentier and Ingmar Lehmann 2009. Mathematical Amazements and Surprises: Fascinating Figures and Noteworthy Numbers. ISBN 978-1-5910-2723-2.