a(n) = floor(n*(n-1)/21).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =1a(7) =2a(8) =2a(9) =3a(10) =4a(11) =5a(12) =6a(13) =7a(14) =8a(15) =10a(16) =11a(17) =12a(18) =14a(19) =16a(20) =18a(21) =20a(22) =22a(23) =24a(24) =26a(25) =28a(26) =30a(27) =33a(28) =36a(29) =38