a(n) = s(n+3)/6, where s is A024731.

Terms

a(0) =1a(1) =1a(2) =1a(3) =7a(4) =13a(5) =19a(6) =61a(7) =145a(8) =301a(9) =745a(10) =1729a(11) =4447a(12) =10585a(13) =25327a(14) =62569a(15) =156049a(16) =382513a(17) =928369a(18) =2288329a(19) =5862367a(20) =14587717a(21) =35951587a(22) =90006229a(23) =231398497a(24) =580975885a(25) =1450005841a(26) =3655134145a(27) =9449479159a(28) =23897176753a(29) =60127243687