a(1)=2; for n > 1, a(n) = 2^(n-2) + (1/(2n-2)) * Sum_{ d divides n-1 } phi(2d)*2^((n-1)/d).

Terms

a(0) =2a(1) =2a(2) =4a(3) =6a(4) =12a(5) =20a(6) =40a(7) =74a(8) =148a(9) =286a(10) =568a(11) =1118a(12) =2228a(13) =4412a(14) =8788a(15) =17480a(16) =34836a(17) =69392a(18) =138388a(19) =275942a(20) =550560a(21) =1098516a(22) =2192572a(23) =4376666a(24) =8738324a(25) =17448308a(26) =34845304a(27) =69594398a(28) =139011816a(29) =277691852