This number is a prime.
The only prime of period fourteen. Note that 909091 contains 9091, the only prime of period ten. [Beedassy]
909091 is a factor of 1010101010101. [Pimentel]
The largest prime factor, (emirp), of any number formed by repeating two times any 7-digit number or seven times any double-digit number. [Loungrides]