9901

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]