a(n) = 7^n+3^n-1.

Terms

a(0) =1a(1) =9a(2) =57a(3) =369a(4) =2481a(5) =17049a(6) =118377a(7) =825729a(8) =5771361a(9) =40373289a(10) =282534297a(11) =1977503889a(12) =13841818641a(13) =96890604729a(14) =678227855817