a(n) = (1/6) * (7^(n+1) - 3*(-1)^n + 2).

Terms

a(0) =1a(1) =9a(2) =57a(3) =401a(4) =2801a(5) =19609a(6) =137257a(7) =960801a(8) =6725601a(9) =47079209a(10) =329554457a(11) =2306881201a(12) =16148168401a(13) =113037178809a(14) =791260251657