a(n) = (4^n - 2^n) / Clausen(n, 1).

Terms

a(0) =0a(1) =1a(2) =2a(3) =28a(4) =8a(5) =496a(6) =96a(7) =8128a(8) =2176a(9) =130816a(10) =15872a(11) =2096128a(12) =6144a(13) =33550336a(14) =44736512a(15) =536854528a(16) =8421376a(17) =8589869056a(18) =86114304a(19) =137438691328a(20) =3331850240a(22) =127479578624a(24) =103104380928