def getisinvertible(self, a=None):
        if not a is None:
            a = a
        else:
            if self.isindependent:
                a = self.reduceform(self.ma)[1:]
            else:
                a = self.ma[1:]
        if a.shape[0] == 0:
            # no ma lags
            self.maeigenvalues = np.array([], np.complex)
            return True


        amat = self.stacksquare(a)
        ev = np.sort(np.linalg.eigvals(amat))[::-1]
        self.maeigenvalues = 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.getisinvertible