def r(o, t):
    """
    Transform back from the Boltzmann variable into `r`.

    Parameters
    ----------
    o : float or numpy.ndarray
        Value(s) of the Boltzmann variable. 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
        `o`. Values must be positive.

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

    See also
    --------
    o
    t
    """
    return o * t ** 0.5
