A sequence of prime under n: parallel version

primes3.fs

let pfilterRange predicate (i, j) =
    let results = ResizeArray(j-i+1)
    let monitor = new Object()
    Parallel.For(
        i, j, new ParallelOptions(),
        (fun () -> ResizeArray(j-i+1)), 
        (fun k _ (localList: ResizeArray<_>) ->                          
            if predicate k then localList.Add(k)
            localList),
        (fun local -> lock (monitor) (fun () -> results.AddRange(local)))
          ) |> ignore
    results.ToArray()

let rec primesUnderParallel = function
    | n when n<=2 -> [||]
    | 3 -> [|2|]
    | n when n<=100 -> primesUnder n
    | n -> let ns = n |> float |> sqrt |> ceil |> int
           let smallers = primesUnderParallel ns            
           Array.append smallers (pfilterRange (indivisible smallers) (ns, n-1))