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Turn this prime number upside down to get the year of the Norman Conquest of England. Any 12digit 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 12digit 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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