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 an array, it must have a shape
        broadcastable with `t`.
    t : float or numpy.ndarray
        Time(s). If an array, it must have a shape broadcastable with `o`.
        Values must not be negative.

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

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