def __discount_PF(i, n):
    """
    Present worth factor

    Factor:  (P/F, i, N)
    Formula: P = F(1+i)^N

    :param i:
    :param n:
    :return:

    Cash Flow:
                 F
                 |
                 |
    --------------
    |
    P
    """
    return (1 + i) ** (-n)