def FpFs(A, Phi, Rp, r):
    """
    Planet-star flux ratio (Equation 11 from Robinson et al. 2016).

    :math:`\\frac{{F}_{p,\\lambda }}{{F}_{{\\rm{s}},\\lambda }}=A{\\rm{\\Phi }}(\\alpha ){\\left(\\displaystyle \\frac{{R}_{{\\rm{p}}}}{r}\\right)}^{2}`

    Parameters
    ----------
    A : float or array-like
        Planetary geometric albedo
    Phi : float
        Planetary phase function
    Rp : float
        Planetary radius [Earth radii]
    r : float
        Planetary orbital semi-major axis [AU]

    Returns
    -------
    FpFs : float or array-like
        Planet-star flux ratio
    """
    Re = 6371000.0
    ds = 149597900000.0
    return A * Phi * (Rp * Re / r / ds) ** 2.0
