code: def z_normalise_series(X):
      """Numba series normalization function for a 1d numpy array.
  
      Parameters
      ----------
      X : 1d numpy array
          A 1d numpy array of values
  
      Returns
      -------
      arr : 1d numpy array
          The normalised series
      """
      s = stats.std(X)
      if s > 0:
          arr = (X - stats.mean(X)) / s
      else:
          arr = X - stats.mean(X)
      return arr
  
file_path: sktime/sktime/utils/numba/general.py
imports: from sktime.utils.validation._dependencies import _check_soft_dependencies
  import numpy
  import sktime.utils.numba.stats
  from sktime.utils.numba.njit import njit
label_desc: Use image node_id 4 for calling a global function with example usage: z_normalise_series(X) and returns: arr