a(n) = phi(n^5 - 1)/5 where phi is A000010.

Terms

a(0) =6a(1) =22a(2) =120a(3) =280a(4) =1240a(5) =1120a(6) =5400a(7) =5280a(8) =12960a(9) =12880a(10) =45240a(11) =24752a(12) =90240a(13) =59160a(14) =96000a(15) =141984a(16) =346880a(17) =163800a(18) =540000a(19) =326720a(20) =588984a(21) =585120a(22) =1523280a(23) =582400a(24) =1728000a(25) =1203840a(26) =2294712a(27) =1758096a(28) =4692408a(29) =1388000