is_prime

let rec multiple_iter n x =
  if x = 2 then true
  else
    if multiple_of n x = true then false
    else multiple_iter n (x - 1);;

let is_prime n =
  if n <> 2 && n mod 2 = 0 || n < 2 then false
  else
    if n = 2 || n = 3 then true
    else
      let root = integer_square_root n in
      multiple_iter n root;;