'check whether the auto-regressive lag-polynomial is stationary
Returns
isinvertible : boolean
*attaches*
maeigenvalues : complex array
eigenvalues sorted by absolute value
References
formula taken from NAG manual'

def getisinvertible(self, a=None):

    if (not (a is None)):
        a = a
    elif self.isindependent:
        a = self.reduceform(self.ma)[1:]
    else:
        a = self.ma[1:]
    if (a.shape[0] == 0):
        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()
