a(n) = floor( n*(n-1)*(n-2)/26 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =4a(7) =8a(8) =12a(9) =19a(10) =27a(11) =38a(12) =50a(13) =66a(14) =84a(15) =105a(16) =129a(17) =156a(18) =188a(19) =223a(20) =263a(21) =306a(22) =355a(23) =408a(24) =467a(25) =530a(26) =600a(27) =675a(28) =756a(29) =843