input: Write a python function to find all prime numbers up to a given number.
  assert find_prime_numbers(10)==[2, 3, 5, 7]
  assert find_prime_numbers(20)==[2, 3, 5, 7, 11, 13, 17, 19]
  assert find_prime_numbers(5)==[2, 3, 5]
output: def find_prime_numbers(n):
      if n < 2:
          return []
      
      primes = []
      for num in range(2, n+1):
          is_prime = True
          for i in range(2, int(num**0.5) + 1):
              if num % i == 0:
                  is_prime = False
                  break
          if is_prime:
              primes.append(num)
      
      return primes