Numerator of sum(i=1..n, 3*i/4^i ).

Terms

a(0) =3a(1) =9a(2) =81a(3) =21a(4) =1359a(5) =2727a(6) =21837a(7) =21843a(8) =349515a(9) =699045a(10) =5592393a(11) =2796201a(12) =89478471a(13) =178956963a(14) =1431655749a(15) =1431655761a(16) =22906492227a(17) =45812984481a(18) =366503875905a(19) =22906492245