a(n) = (2^n - 1)^2.

Terms

a(0) =1a(1) =9a(2) =49a(3) =225a(4) =961a(5) =3969a(6) =16129a(7) =65025a(8) =261121a(9) =1046529a(10) =4190209a(11) =16769025a(12) =67092481a(13) =268402689a(14) =1073676289a(15) =4294836225a(16) =17179607041a(17) =68718952449a(18) =274876858369