a(n) = s(n+3)/6, where s is A024743.

Terms

a(0) =1a(1) =3a(2) =9a(3) =29a(4) =93a(5) =297a(6) =949a(7) =3195a(8) =10701a(9) =35675a(10) =118761a(11) =410949a(12) =1413385a(13) =4835421a(14) =16526241a(15) =58096283a(16) =203257713a(17) =708000147a(18) =2462308561a(19) =8753612709a(20) =30980135637a(21) =109194529241a(22) =384438181581