a(n) = (8^n - 1)*4/7.

Terms

a(0) =0a(1) =4a(2) =36a(3) =292a(4) =2340a(5) =18724a(6) =149796a(7) =1198372a(8) =9586980a(9) =76695844a(10) =613566756a(11) =4908534052a(12) =39268272420a(13) =314146179364