G.f.: g(x) * g(x^2), where g(x) is the g.f. of A000081.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =1a(5) =3a(6) =5a(7) =13a(8) =26a(9) =65a(10) =147a(11) =369a(12) =899a(13) =2298a(14) =5851a(15) =15261a(16) =39945a(17) =105948a(18) =282504a(19) =759480a(20) =2052027a(21) =5576017a(22) =15216998a(23) =41705762a(24) =114715503a(25) =316611401a(26) =876466003a(27) =2433091773a(28) =6771462322a(29) =18889829555