a(n) = floor( n(n-1)/26 ).

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) =5a(13) =6a(14) =7a(15) =8a(16) =9a(17) =10a(18) =11a(19) =13a(20) =14a(21) =16a(22) =17a(23) =19a(24) =21a(25) =23a(26) =25a(27) =27a(28) =29a(29) =31