def getisstationary(self, a=None):
        if not a is None:
            a = a
        else:
            if self.isstructured:
                a = -self.reduceform(self.ar)[1:]
            else:
                a = -self.ar[1:]
        amat = self.stacksquare(a)
        ev = np.sort(np.linalg.eigvals(amat))[::-1]
        self.areigenvalues = ev
        return (np.abs(ev) < 1).all()
check whether the auto-regressive lag-polynomial is stationary
dataset/ETHPy150Open statsmodels/statsmodels/statsmodels/tsa/varma_process.py/VarmaPoly.getisstationary