10838689

This number is a prime.

Any 21-digit number that has any set of 3 digits repeating 7 times (abcabcabcabcabcabcabc) or any set of 7 digits repeating 3 times (abcdefgabcdefgabcdefg) including any 21-digit repdigit has 10838689 as a highest prime factor. [Schuler]