a(2n) = A000045(6n) + 1, a(2n+1) = A000045(6n+3) - 1.

Terms

a(0) =1a(1) =1a(2) =9a(3) =33a(4) =145a(5) =609a(6) =2585a(7) =10945a(8) =46369a(9) =196417a(10) =832041a(11) =3524577a(12) =14930353a(13) =63245985a(14) =267914297a(15) =1134903169a(16) =4807526977a(17) =20365011073a(18) =86267571273a(19) =365435296161