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

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =2a(6) =6a(7) =14a(8) =37a(9) =90a(10) =232a(11) =584a(12) =1512a(13) =3906a(14) =10246a(15) =26984a(16) =71766a(17) =191852a(18) =516400a(19) =1396760a(20) =3797435a(21) =10367628a(22) =28420466a(23) =78183462a(24) =215791426a(25) =597368222a(26) =1658233794a(27) =4614679792a(28) =12872125836a(29) =35982713314