99990001

This number is a prime.

The largest prime factor p of any 24-digit repdigit number is also the largest prime factor of any 2-digit number repeated twelve times, of any 3-digit number repeated eight times, of any 4-digit number repeated six times and of any 6-digit number repeated four times. Note that p is also a prime, (but not always the largest), for any 24-digit number formed from an 8-digit number repeated three times and from a 12-digit number repeated two times. [Loungrides]