a(n) = A002703(n) + 2.

Terms

a(0) =2a(1) =2a(2) =2a(3) =4a(4) =8a(5) =16a(6) =26a(7) =48a(8) =90a(9) =164a(10) =302a(11) =564a(12) =1058a(13) =1984a(14) =3744a(15) =7084a(16) =13440a(17) =25576a(18) =48770a(19) =93200a(20) =178482a(21) =342394a(22) =657920a(23) =1266204a(24) =2440320a(25) =4709376a(26) =9099506a(27) =17602324a(28) =34087012a(29) =66076416