167772161

This number is a prime.

The prime number 167772161 which is equal to 2^25*5+1 was found in 1878 to be a factor of 2^(2^23)+1 or Fermat 23 by Pervushin (also spelled Pervouchine). Note that it was found 125 years later in 2003 by Dario Alpern to be a factor of 10^(10^100)-1 or a googolplex minus one. [Dobb]