a(n)=2*(4^n-1)/denominator(B(2n)) where B(k) denotes the k-th Bernoulli number.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =17a(5) =31a(6) =3a(7) =5461a(8) =257a(9) =657a(10) =6355a(11) =60787a(12) =12291a(13) =22369621a(14) =617093a(15) =149943a(16) =16843009a(17) =5726623061a(18) =71613a(19) =91625968981a(20) =162529435a(21) =4870483401a(22) =50991843607a(23) =499069107643a(24) =12129927891