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

Terms

a(0) =3a(1) =10a(2) =30a(3) =94a(4) =318a(5) =1150a(6) =4350a(7) =16894a(8) =66558a(9) =264190a(10) =1052670a(11) =4202494a(12) =16793598a(13) =67141630a(14) =268500990a(15) =1073872894a(16) =4295229438a(17) =17180393470a(18) =68720525310a(19) =274880004094