a(n) = floor(n/5)*floor((n+1)/5)*floor((n+2)/5)*floor((n+3)/5)*floor((n+4)/5).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =2a(7) =4a(8) =8a(9) =16a(10) =32a(11) =48a(12) =72a(13) =108a(14) =162a(15) =243a(16) =324a(17) =432a(18) =576a(19) =768a(20) =1024a(21) =1280a(22) =1600a(23) =2000a(24) =2500a(25) =3125a(26) =3750a(27) =4500a(28) =5400a(29) =6480