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

Terms

a(0) =2a(1) =14a(2) =98a(3) =686a(4) =4802a(5) =33614a(6) =235298a(7) =1647086a(8) =11529602a(9) =80707214a(10) =564950498a(11) =3954653486a(12) =27682574402a(13) =193778020814