let getLPF num =
    let rec getLPF l rest =
        seq { yield 2L; yield! seq {3L..2L..(rest |> float |> sqrt |> int64)}}
        |> Seq.tryFind (fun d -> rest % d = 0L)
        |> function
           | None -> max l rest
           | Some d -> getLPF (max l d) (rest / d)
    getLPF 1L num;;

getLPF 600851475143L