[ n(n-1)/18 ].

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =1a(7) =2a(8) =3a(9) =4a(10) =5a(11) =6a(12) =7a(13) =8a(14) =10a(15) =11a(16) =13a(17) =15a(18) =17a(19) =19a(20) =21a(21) =23a(22) =25a(23) =28a(24) =30a(25) =33a(26) =36a(27) =39a(28) =42a(29) =45