class_docstr: class to keep track of Varma polynomial format
  
  
  Examples
  --------
  
  ar23 = np.array([[[ 1. ,  0. ],
                   [ 0. ,  1. ]],
  
                  [[-0.6,  0. ],
                   [ 0.2, -0.6]],
  
                  [[-0.1,  0. ],
                   [ 0.1, -0.1]]])
  
  ma22 = np.array([[[ 1. ,  0. ],
                   [ 0. ,  1. ]],
  
                  [[ 0.4,  0. ],
                   [ 0.2, 0.3]]])
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docstr: check whether the auto-regressive lag-polynomial is stationary
  
  Returns
  -------
  isinvertible : bool
  
  *attaches*
  
  maeigenvalues : complex array
      eigenvalues sorted by absolute value
  
  References
  ----------
  formula taken from NAG manual
docstr_tok: ["check","whether","the","auto-regressive","lag-polynomial","is","stationary","Returns","--","--","--","-","isinvertible",":","bool","*","attaches","*","maeigenvalues",":","complex","array","eigenvalues","sorted","by","absolute","value","References","--","--","--","--","--","formula","taken","from","NAG","manual"]
code: def getisinvertible(self, a=None):
      """check whether the auto-regressive lag-polynomial is stationary
  
      Returns
      -------
      isinvertible : bool
  
      *attaches*
  
      maeigenvalues : complex array
          eigenvalues sorted by absolute value
  
      References
      ----------
      formula taken from NAG manual
  
      """
      if a is not 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()
  
imports: import numpy
  from scipy import signal
  from statsmodels.tsa.tsatools import lagmat
label_desc: Use image node_id 8 for calling the VarmaPoly obj's underlying member method code with example usage: obj.getisinvertible(a) and returns: unknown, True