(2^p-1)^2 where p is prime.A174744(2^p-1)^2 where p is prime.Terms a(0) =9a(1) =49a(2) =961a(3) =16129a(4) =4190209a(5) =67092481a(6) =17179607041a(7) =274876858369External referencesoeis: A174744