a(n) = 1+4^(n+1)-4*(-2)^n.

Terms

a(0) =1a(1) =25a(2) =49a(3) =289a(4) =961a(5) =4225a(6) =16129a(7) =66049a(8) =261121a(9) =1050625a(10) =4190209a(11) =16785409a(12) =67092481a(13) =268468225a(14) =1073676289a(15) =4295098369a(16) =17179607041a(17) =68720001025a(18) =274876858369