a(n) = (5*4^n - 2)/3.

Terms

a(0) =1a(1) =6a(2) =26a(3) =106a(4) =426a(5) =1706a(6) =6826a(7) =27306a(8) =109226a(9) =436906a(10) =1747626a(11) =6990506a(12) =27962026a(13) =111848106a(14) =447392426a(15) =1789569706a(16) =7158278826a(17) =28633115306a(18) =114532461226a(19) =458129844906