a(0)=3; for n > 0, a(n) = 2^(2*n) + 3*2^(n-1) + 1.

Terms

a(0) =3a(1) =8a(2) =23a(3) =77a(4) =281a(5) =1073a(6) =4193a(7) =16577a(8) =65921a(9) =262913a(10) =1050113a(11) =4197377a(12) =16783361a(13) =67121153a(14) =268460033a(15) =1073790977a(16) =4295065601a(17) =17180065793a(18) =68719869953a(19) =274878693377