a(n) = floor(n/5)*ceiling(n/5).

Terms

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