a(n) = (2*5^n + (-1)^n)/3.

Terms

a(0) =1a(1) =3a(2) =17a(3) =83a(4) =417a(5) =2083a(6) =10417a(7) =52083a(8) =260417a(9) =1302083a(10) =6510417a(11) =32552083a(12) =162760417a(13) =813802083a(14) =4069010417a(15) =20345052083a(16) =101725260417a(17) =508626302083