a(n) = 3*2^floor((n-1)/2) + (-1)^n.

Terms

a(0) =2a(1) =4a(2) =5a(3) =7a(4) =11a(5) =13a(6) =23a(7) =25a(8) =47a(9) =49a(10) =95a(11) =97a(12) =191a(13) =193a(14) =383a(15) =385a(16) =767a(17) =769a(18) =1535a(19) =1537a(20) =3071a(21) =3073a(22) =6143a(23) =6145a(24) =12287a(25) =12289a(26) =24575a(27) =24577a(28) =49151a(29) =49153