9091

This number is a prime.

A unique prime indeed. Reverse the order of its middle digits to form the next unique prime (9901).

The greatest prime factor of any 10-digit repdigit is 9091. [La Haye]

The smallest prime of the form 10k-10^(k-1).....+10^0, for k=4. [Silva]

9091, 909091, 909090909090909091, and 909090909090909090909090909091 are each prime, and 9091 and 909091 are the two smallest prime factors of 9090909090909090909090909090909091. Submitted on 9/09/09. [Green]

The lesser in the only pair of primes, (9091, 9293), formed from four successive double-digit composite numbers, 90, 91, 92, 93. [Loungrides]