Number of even sequences with period 2n (bisection of A000011).

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =18a(5) =44a(6) =122a(7) =362a(8) =1162a(9) =3914a(10) =13648a(11) =48734a(12) =176906a(13) =649532a(14) =2405236a(15) =8964800a(16) =33588234a(17) =126390032a(18) =477353376a(19) =1808676326a(20) =6872485104a(21) =26179922024a(22) =99957747388a(23) =382443112538