a(n) = Card{ (x,y,z,u) | lcm(x,y,z,u)=n }.

Terms

a(0) =1a(1) =15a(2) =15a(3) =65a(4) =15a(5) =225a(6) =15a(7) =175a(8) =65a(9) =225a(10) =15a(11) =975a(12) =15a(13) =225a(14) =225a(15) =369a(16) =15a(17) =975a(18) =15a(19) =975a(20) =225a(21) =225a(22) =15a(23) =2625a(24) =65a(25) =225a(26) =175a(27) =975a(28) =15a(29) =3375