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

Terms

a(0) =2a(1) =20a(2) =146a(3) =1028a(4) =7202a(5) =50420a(6) =352946a(7) =2470628a(8) =17294402a(9) =121060820a(10) =847425746a(11) =5931980228a(12) =41523861602a(13) =290667031220