code: def _get_acf(X, ca):
      acf = np.zeros(len(X))
      if ca[0].real != 0:
          for i in range(len(X)):
              acf[i] = ca[i].real / ca[0].real
      return acf
  
imports: import math
  import numpy
  from sktime.utils.numba.njit import njit
label_desc: Use image node_id 6 for calling a global function with example usage: _get_acf(X, ca) and returns: acf