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

Terms

a(0) =3a(1) =27a(2) =195a(3) =1371a(4) =9603a(5) =67227a(6) =470595a(7) =3294171a(8) =23059203a(9) =161414427a(10) =1129900995a(11) =7909306971a(12) =55365148803a(13) =387556041627