a(n) = (7^n - 1)/2.

Terms

a(0) =0a(1) =3a(2) =24a(3) =171a(4) =1200a(5) =8403a(6) =58824a(7) =411771a(8) =2882400a(9) =20176803a(10) =141237624a(11) =988663371a(12) =6920643600a(13) =48444505203a(14) =339111536424