[ n(n-1)/19 ].

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =1a(7) =2a(8) =2a(9) =3a(10) =4a(11) =5a(12) =6a(13) =8a(14) =9a(15) =11a(16) =12a(17) =14a(18) =16a(19) =18a(20) =20a(21) =22a(22) =24a(23) =26a(24) =29a(25) =31a(26) =34a(27) =36a(28) =39a(29) =42