code: def _CO_FirstMin_ac(X_ac):
      # First minimum of autocorrelation function.
      for i in range(1, len(X_ac) - 1):
          if X_ac[i] < X_ac[i - 1] and X_ac[i] < X_ac[i + 1]:
              return i
      return len(X_ac)
  
imports: import math
  import numpy
  from sktime.utils.numba.njit import njit
label_desc: Use image node_id 21 for calling a global function with example usage: _CO_FirstMin_ac(X_ac) and returns: len, i