(2^(p-1)-1)/(3*p) where p = prime(n).

Terms

a(0) =1a(1) =3a(2) =31a(3) =105a(4) =1285a(5) =4599a(6) =60787a(7) =3085465a(8) =11545611a(9) =619094385a(10) =8939118925a(11) =34093383807a(12) =499069107643