a(n) = floor(n*(n-1)/27).

Terms

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