A sequence of prime under n

primes1.fs

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