(3*7^n+1)/2.

Terms

a(0) =2a(1) =11a(2) =74a(3) =515a(4) =3602a(5) =25211a(6) =176474a(7) =1235315a(8) =8647202a(9) =60530411a(10) =423712874a(11) =2965990115a(12) =20761930802a(13) =145333515611