a(n) = phi(n^3 - 1)/3 where phi is A000010.

Terms

a(0) =2a(1) =4a(2) =12a(3) =20a(4) =56a(5) =36a(6) =144a(7) =96a(8) =216a(9) =144a(10) =520a(11) =240a(12) =840a(13) =480a(14) =576a(15) =816a(16) =1568a(17) =756a(18) =2520a(19) =1232a(20) =1872a(21) =1560a(22) =4400a(23) =1440a(24) =4320a(25) =3024a(26) =4860a(27) =3168a(28) =7056a(29) =2640