def is_stationary_adf(y):
    """
    Whether a time series y is stationary or not. Uses the Augmented Dickey-Fuller Test.

    :param y: A 1D np.array
    :returns: BOOLEAN
    """
    import statsmodels.tsa.stattools as smtools
    adf = smtools.adfuller(y)
    return adf[0] < adf[4]['5%']
