Number of even sequences with period 2n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =4a(4) =12a(5) =28a(6) =94a(7) =298a(8) =1044a(9) =3658a(10) =13164a(11) =47710a(12) =174948a(13) =645436a(14) =2397342a(15) =8948416a(16) =33556500a(17) =126324496a(18) =477225962a(19) =1808414182a(20) =6871973952a(21) =26178873448a(22) =99955697946a(23) =382438918234