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

Terms

a(0) =1a(1) =7a(2) =37a(3) =187a(4) =937a(5) =4687a(6) =23437a(7) =117187a(8) =585937a(9) =2929687a(10) =14648437a(11) =73242187a(12) =366210937a(13) =1831054687a(14) =9155273437a(15) =45776367187a(16) =228881835937