a(n) = A027052(n, 2n-9).

Terms

a(0) =0a(1) =2a(2) =6a(3) =20a(4) =68a(5) =230a(6) =776a(7) =2604a(8) =8670a(9) =28606a(10) =93494a(11) =302748a(12) =971810a(13) =3094486a(14) =9782092a(15) =30721056a(16) =95919714a(17) =297938906a(18) =921183940a(19) =2836545972a(20) =8702745304a(21) =26614653494a(22) =81159163058a(23) =246855070144a(24) =749123740876