a(n) = floor(n*(n-1)/22).

Terms

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