a(n) = floor(n/5)*floor((n+1)/5)*floor((n+2)/5).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =1a(7) =1a(8) =2a(9) =4a(10) =8a(11) =8a(12) =8a(13) =12a(14) =18a(15) =27a(16) =27a(17) =27a(18) =36a(19) =48a(20) =64a(21) =64a(22) =64a(23) =80a(24) =100a(25) =125a(26) =125a(27) =125a(28) =150a(29) =180