(s(n)+s(n+1))/18, where s()=A006521.

Terms

a(0) =2a(1) =6a(2) =14a(3) =23a(4) =42a(5) =69a(6) =126a(7) =207a(8) =302a(9) =437a(10) =621a(11) =906a(12) =1275a(13) =1503a(14) =1863a(15) =2718a(16) =3825a(17) =4509a(18) =5589a(19) =6710a(20) =8303a(21) =10298a(22) =12026a(23) =13527a(24) =16767a(25) =20130a(26) =24225a(27) =28557a(28) =30894a(29) =35970