(s(n)+s(n+1))/6, where s()=A006521.

Terms

a(0) =2a(1) =6a(2) =18a(3) =42a(4) =69a(5) =126a(6) =207a(7) =378a(8) =621a(9) =906a(10) =1311a(11) =1863a(12) =2718a(13) =3825a(14) =4509a(15) =5589a(16) =8154a(17) =11475a(18) =13527a(19) =16767a(20) =20130a(21) =24909a(22) =30894a(23) =36078a(24) =40581a(25) =50301a(26) =60390a(27) =72675a(28) =85671a(29) =92682