code: def arma_acf(ar, ma, lags=10):
      """
      Theoretical autocorrelation function of an ARMA process.
  
      Parameters
      ----------
      ar : array_like
          Coefficients for autoregressive lag polynomial, including zero lag.
      ma : array_like
          Coefficients for moving-average lag polynomial, including zero lag.
      lags : int
          The number of terms (lags plus zero lag) to include in returned acf.
  
      Returns
      -------
      ndarray
          The autocorrelations of ARMA process given by ar and ma.
  
      See Also
      --------
      arma_acovf : Autocovariances from ARMA processes.
      acf : Sample autocorrelation function estimation.
      acovf : Sample autocovariance function estimation.
      """
      acovf = arma_acovf(ar, ma, lags)
      return acovf / acovf[0]
  
imports: from statsmodels.compat.numpy import NP_LT_2
  from statsmodels.compat.pandas import Appender
  import warnings
  import numpy
  from scipy import linalg, optimize, signal
  from statsmodels.tools.docstring import Docstring, remove_parameters
  from statsmodels.tools.validation import array_like
label_desc: Use image node_id 3 for calling a global function with example usage: arma_acf(ar, ma, lags) and returns: unknown