code: def acovf_arma11(ar, ma):
      # ARMA(1,1)
      # Florens et al page 278
      # wrong result ?
      # new calculation bigJudge p 311, now the same
      a = -ar[1]
      b = ma[1]
      # rho = [1.]
      # rho.append((1-a*b)*(a-b)/(1.+a**2-2*a*b))
      rho = [(1.0 + b**2 + 2 * a * b) / (1.0 - a**2)]
      rho.append((1 + a * b) * (a + b) / (1.0 - a**2))
      for _ in range(8):
          last = rho[-1]
          rho.append(a * last)
      return np.array(rho)
  
imports: import numpy
  from numpy.testing import assert_array_almost_equal
  import matplotlib.pyplot
  from statsmodels import regression
  from statsmodels.tsa.arima_process import arma_generate_sample, arma_impulse_response
  from statsmodels.tsa.arima_process import arma_acovf, arma_acf
  from statsmodels.tsa.arima.model import ARIMA
  from statsmodels.tsa.stattools import acf, acovf
  from statsmodels.graphics.tsaplots import plot_acf
label_desc: Use image node_id 7 for calling a global function with example usage: acovf_arma11(ar, ma) and returns: np