a(n) = (1/2)*s(n+3), where s = A025251.

Terms

a(0) =0a(1) =1a(2) =2a(3) =1a(4) =6a(5) =9a(6) =12a(7) =41a(8) =60a(9) =121a(10) =310a(11) =505a(12) =1162a(13) =2577a(14) =4760a(15) =11089a(16) =23256a(17) =47089a(18) =107274a(19) =223345a(20) =476366a(21) =1061017a(22) =2237796a(23) =4888313a(24) =10745748a(25) =23048169a(26) =50792638a(27) =111180265a(28) =241786898a(29) =534219297