let isqrt n = n |> float_of_int |> sqrt |> int_of_float;;

let sieve n =
  let is_prime = Array.make n true in
  for i = 2 to isqrt n do
    if is_prime.(i) then
      let j = ref i in
      while i * !j < n do
        is_prime.(i * !j) <- false;
        incr j
      done
  done;
  is_prime.(0) <- false;
  is_prime.(1) <- false;
  is_prime |> Array.to_list |> List.mapi (fun i b -> (i, b)) |>
  List.filter_map (fun (i, b) -> if b then Some i else None);;