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

Terms

a(0) =2a(1) =8a(2) =38a(3) =188a(4) =938a(5) =4688a(6) =23438a(7) =117188a(8) =585938a(9) =2929688a(10) =14648438a(11) =73242188a(12) =366210938a(13) =1831054688a(14) =9155273438a(15) =45776367188a(16) =228881835938