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

Terms

a(0) =1a(1) =3a(2) =25a(3) =75a(4) =625a(5) =1875a(6) =15625a(7) =46875a(8) =390625a(9) =1171875a(10) =9765625a(11) =29296875a(12) =244140625a(13) =732421875a(14) =6103515625a(15) =18310546875a(16) =152587890625a(17) =457763671875