731

This number is a composite.

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]