731 is composite, being the product of the primes 17 and 43, but 7331, 73331, 733331 and 7333331 are all prime. This is the only three-digit composite with this property.
The largest composite number such that deleting any
digit(s) will never yield another composite number, i.e.
only yields primes or 1. [Hasler]
(There is one curio for this number that has not yet been approved by an editor.)