a(n) = bcd, where n = C(b,3)+C(c,2)+C(d,1), b>c>d>=0.

Terms

a(0) =310a(1) =320a(2) =321a(3) =410a(4) =420a(5) =421a(6) =430a(7) =431a(8) =432a(9) =510a(10) =520a(11) =521a(12) =530a(13) =531a(14) =532a(15) =540a(16) =541a(17) =542a(18) =543a(19) =610a(20) =620a(21) =621a(22) =630a(23) =631a(24) =632a(25) =640a(26) =641a(27) =642a(28) =643a(29) =650