def dustSurfaceDensityDouble(R, Sig1, Sig2, R1, p1, p2):
    """
    Calculates the dust surface density (Sigma d) from a 2-sloped discontinuous power law.
    """
    if R <= R1:
        return Sig1 * pow(R / R1, -p1)
    else:
        return Sig2 * pow(R / R1, -p2)