a(n) = (7*9^n + 1)/4.

Terms

a(0) =2a(1) =16a(2) =142a(3) =1276a(4) =11482a(5) =103336a(6) =930022a(7) =8370196a(8) =75331762a(9) =677985856a(10) =6101872702a(11) =54916854316a(12) =494251688842