'Back-transforms the Box-Cox transformed data array, by means of the
indicated method. The provided argument lmbda should be the lambda
parameter that was used to initially transform the data.
Parameters
x : array_like
The transformed series.
lmbda : float
The lambda parameter that was used to transform the series.
method : {\'naive\'}
Indicates the method to be used in the untransformation. Defaults
to \'naive\', which reverses the transformation.
NOTE: \'naive\' is implemented natively, while other methods may be
available in subclasses!
Returns
y : array_like
The untransformed series.'

def untransform_boxcox(self, x, lmbda, method='naive'):

    method = method.lower()
    x = np.asarray(x)
    if (method == 'naive'):
        if np.isclose(lmbda, 0.0):
            y = np.exp(x)
        else:
            y = np.power(((lmbda * x) + 1), (1.0 / lmbda))
    else:
        raise ValueError("Method    '{0}'    not    understood.".format(method))
    return y
