Number of even sequences with period 2n (bisection of A000013).

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =20a(5) =56a(6) =180a(7) =596a(8) =2068a(9) =7316a(10) =26272a(11) =95420a(12) =349716a(13) =1290872a(14) =4794088a(15) =17896832a(16) =67110932a(17) =252648992a(18) =954444608a(19) =3616828364a(20) =13743921632a(21) =52357746896a(22) =199911300472a(23) =764877836468