a(n) = A024741(n+3)/6.

Terms

a(0) =1a(1) =2a(2) =4a(3) =11a(4) =28a(5) =68a(6) =169a(7) =470a(8) =1276a(9) =3395a(10) =9016a(11) =25736a(12) =71785a(13) =197084a(14) =543136a(15) =1562987a(16) =4441888a(17) =12490268a(18) =35118241a(19) =102094436a(20) =293497252a(21) =836212259a(22) =2384733976a(23) =6972232316a(24) =20220310225a(25) =58240787582a(26) =167689305760a(27) =493029967907