a(n) = (11*5^n + 1)/4.

Terms

a(0) =3a(1) =14a(2) =69a(3) =344a(4) =1719a(5) =8594a(6) =42969a(7) =214844a(8) =1074219a(9) =5371094a(10) =26855469a(11) =134277344a(12) =671386719a(13) =3356933594a(14) =16784667969a(15) =83923339844a(16) =419616699219