21

This number is a composite.

Single Curio View: (Seek other curios for this number)

21 is the only known multidigit number m such that both numbers m * m! - prime(m) and m * m! + prime(m) are primes. [Firoozbakht]

Submitted: 2004-09-11 08:35:00; Last Modified: 2008-09-23 02:22:12.