floor(n/5)*floor((n+1)/5)*floor((n+2)/5)*floor((n+3)/5).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =1a(7) =2a(8) =4a(9) =8a(10) =16a(11) =16a(12) =24a(13) =36a(14) =54a(15) =81a(16) =81a(17) =108a(18) =144a(19) =192a(20) =256a(21) =256a(22) =320a(23) =400a(24) =500a(25) =625a(26) =625a(27) =750a(28) =900a(29) =1080