a(n) = floor(n/6)*ceiling(n/6).

Terms

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