a(n) = 2*7^n - 1.

Terms

a(0) =1a(1) =13a(2) =97a(3) =685a(4) =4801a(5) =33613a(6) =235297a(7) =1647085a(8) =11529601a(9) =80707213a(10) =564950497a(11) =3954653485a(12) =27682574401a(13) =193778020813