a(n) = floor(n/5)*floor((n+1)/5).

Terms

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