input: def fibonacci_series_up_to_n(n: int) -> str:
      """Prints the Fibonacci series up to a given number n and returns a string representation of the series.
      >>> fibonacci_series_up_to_n(5)
      '0 1 1 2 3'
      >>> fibonacci_series_up_to_n(10)
      '0 1 1 2 3 5 8'
      """
      a, b = 0, 1
      series = []
      while a <= n:
          series.append(str(a))
          a, b = b, a + b
      return ' '.join(series)
output:     a, b = 0, 1
      series = []
      while a <= n:
          series.append(str(a))
          a, b = b, a + b
      return ' '.join(series)