a(n) = T(n,n+3), array T as in A050143.

Terms

a(0) =0a(1) =1a(2) =7a(3) =42a(4) =242a(5) =1375a(6) =7773a(7) =43876a(8) =247684a(9) =1399293a(10) =7913955a(11) =44812878a(12) =254064726a(13) =1442131899a(14) =8195232633a(15) =46621424520a(16) =265490365448