This number is a prime.

+ Turn this prime number upside down to get the year of the Norman Conquest of England.

+ Any 12-digit number that has three or four digits that repeat themselves throughout the number will have 9901 as a prime factor. [Schuler]

+ 9901, 99990001, 999999000001, and 9999999900000001, are primes. [Avrutin]

+ 9901 divides 111111111111111111111111. [Avrutin]

+ The greatest prime factor of any 12-digit repdigit is 9901. [La Haye]

+ A prime whose square (98029801) is the concatenation of two consecutive numbers in descending order. [Bopardikar]

+ 9901=99^2+99^1+99^0. [Silva]

+ The only known prime with reciprocal of period dozen. [Post]

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

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