This number is a prime.
A unique prime indeed. Reverse the order of its middle digits to form the next unique prime (9901).
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]