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

Terms

a(0) =3a(1) =15a(2) =99a(3) =687a(4) =4803a(5) =33615a(6) =235299a(7) =1647087a(8) =11529603a(9) =80707215a(10) =564950499a(11) =3954653487a(12) =27682574403a(13) =193778020815