a(n) = (3*11^n + 3^n)/4.

Terms

a(0) =1a(1) =9a(2) =93a(3) =1005a(4) =11001a(5) =120849a(6) =1328853a(7) =14615925a(8) =160770801a(9) =1768465689a(10) =19453083213a(11) =213983797245