a(n) = (5*4^n + 1)/3.

Terms

a(0) =2a(1) =7a(2) =27a(3) =107a(4) =427a(5) =1707a(6) =6827a(7) =27307a(8) =109227a(9) =436907a(10) =1747627a(11) =6990507a(12) =27962027a(13) =111848107a(14) =447392427a(15) =1789569707a(16) =7158278827a(17) =28633115307a(18) =114532461227a(19) =458129844907