let lnPrec x eps =
    Seq.unfold (fun (s, n) ->
                    if (s/(float n) < eps) then None
                    else Some( (s, n), (s*(x-1.0)/x, n+1) )) ((x-1.0)/x, 1)
    |> Seq.fold (fun acc (s, n) -> acc + s/(float n)) 0.0

let lnRange x range =
    Seq.unfold (fun (s, n) ->
                    if (n > range) then None
                    else Some( (s, n), (s*(x-1.0)/x, n+1) )) ((x-1.0)/x, 1)
    |> Seq.fold (fun acc (s,n) -> acc + s/(float n)) 0.0