def Frr_unit_sphere(r):
    """Antiderivative of RR_sphere.

    The RR from r1 to r2 for a unit spherical area is
    rr_unit_sphere(r2) - rr_unit_sphere(r1)
    """
    return 3 * r**3/3 - 9 / 4 * r**4/4 + 3 / 16 * r**6/6