a(n) = (11*7^n + 1)/6.

Terms

a(0) =2a(1) =13a(2) =90a(3) =629a(4) =4402a(5) =30813a(6) =215690a(7) =1509829a(8) =10568802a(9) =73981613a(10) =517871290a(11) =3625099029a(12) =25375693202a(13) =177629852413