a(0)=4; if n > 0 is even then a(n) = 2^(n/2+1)+3, otherwise a(n) = 3*(2^((n-1)/2)+1).

Terms

a(0) =4a(1) =6a(2) =7a(3) =9a(4) =11a(5) =15a(6) =19a(7) =27a(8) =35a(9) =51a(10) =67a(11) =99a(12) =131a(13) =195a(14) =259a(15) =387a(16) =515a(17) =771a(18) =1027a(19) =1539a(20) =2051a(21) =3075a(22) =4099a(23) =6147a(24) =8195a(25) =12291a(26) =16387a(27) =24579a(28) =32771a(29) =49155