a(n) = (1/2) * A027052(n, 2n-1).

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =21a(5) =51a(6) =128a(7) =329a(8) =861a(9) =2285a(10) =6132a(11) =16606a(12) =45313a(13) =124446a(14) =343680a(15) =953753a(16) =2658133a(17) =7436541a(18) =20875972a(19) =58783892a(20) =165989825a(21) =469903672a(22) =1333359488a(23) =3791535934a(24) =10802911297a(25) =30836181436a(26) =88169413364a(27) =252500533673a(28) =724182805389