This number is a prime.

+ 99990001 = 9999^2 + 9999^1 + 9999^0. [Nwamba]

+ 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]

(There is one curio for this number that has not yet been approved by an editor.)

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