def do_dr(r, t):
    """
    Spatial derivative of the Boltzmann transformation.

    Returns the partial derivative :math:`\\partial o/\\partial r` evaluated at
    (`r`, `t`).

    Parameters
    ----------
    r : float or numpy.ndarray
        Location(s). If a `numpy.ndarray`, it must have a shape broadcastable
        with `t`.
    t : float or numpy.ndarray
        Time(s). If a `numpy.ndarray`, it must have a shape broadcastable with
        `r`. Values must be positive.

    Returns
    -------
    do_dr : float or numpy.ndarray
        The return is a float if both `r` and `t` are floats. Otherwise it is a
        `numpy.ndarray` of the shape that results from broadcasting `r` and
        `t`.

    See also
    --------
    o
    do_dt
    """
    return 1 / t ** 0.5
