a(n) = A027052(n, 2n-2).

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =23a(5) =59a(6) =153a(7) =401a(8) =1063a(9) =2847a(10) =7693a(11) =20947a(12) =57413a(13) =158265a(14) =438467a(15) =1220145a(16) =3408759a(17) =9556815a(18) =26878861a(19) =75815839a(20) =214411865a(21) =607827693a(22) =1726911631a(23) =4916352891a(24) =14022750725a(25) =40066540277a(26) =114666463855a(27) =328662240617