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

Terms

a(0) =4a(1) =11a(2) =31a(3) =95a(4) =319a(5) =1151a(6) =4351a(7) =16895a(8) =66559a(9) =264191a(10) =1052671a(11) =4202495a(12) =16793599a(13) =67141631a(14) =268500991a(15) =1073872895a(16) =4295229439a(17) =17180393471a(18) =68720525311a(19) =274880004095