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

Terms

a(0) =5a(1) =25a(2) =105a(3) =425a(4) =1705a(5) =6825a(6) =27305a(7) =109225a(8) =436905a(9) =1747625a(10) =6990505a(11) =27962025a(12) =111848105a(13) =447392425a(14) =1789569705a(15) =7158278825a(16) =28633115305a(17) =114532461225a(18) =458129844905