a(n) = floor(n*(n-1)/30).

Terms

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