a(n) = floor(n*(n-1)/15).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =2a(7) =2a(8) =3a(9) =4a(10) =6a(11) =7a(12) =8a(13) =10a(14) =12a(15) =14a(16) =16a(17) =18a(18) =20a(19) =22a(20) =25a(21) =28a(22) =30a(23) =33a(24) =36a(25) =40a(26) =43a(27) =46a(28) =50a(29) =54