def    fvga(a, i, g, n):
    """ This function is for the future value of an annuity
    with growth rate.  It is the future value of a growing
    stream of periodic investments.
    a = Periodic Investment (1000)
    i = interest rate as decimal (.0675)
    g = the growth rate (.05)
    n = the number of compound periods (20)
    Example: fv(1000, .0675, .05, 20)
    """
    return a * ((((1 + i) ** n) - (((1 + g) ** n)))/(i - g))