def o(r, t):
    """
    Transform to the Boltzmann variable.

    Returns the Boltzmann variable at the given `r` and `t`, which is the
    result of applying the Boltzmann transformation:

    .. math::
        o(r,t) = r/\\sqrt 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
    -------
    o : 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
    --------
    do_dr
    do_dt
    r
    t
    as_o
    """
    return r / t ** 0.5
