code: def _zseries_mul(z1, z2):
      """Multiply two z-series.
  
      Multiply two z-series to produce a z-series.
  
      Parameters
      ----------
      z1, z2 : 1-D ndarray
          The arrays must be 1-D but this is not checked.
  
      Returns
      -------
      product : 1-D ndarray
          The product z-series.
  
      Notes
      -----
      This is simply convolution. If symmetric/anti-symmetric z-series are
      denoted by S/A then the following rules apply:
  
      S*S, A*A -> S
      S*A, A*S -> A
  
      """
      return np.convolve(z1, z2)
  
imports: import numpy
  import numpy.linalg
  from numpy.core.multiarray import normalize_axis_index
  from .None import polyutils
  from ._polybase import ABCPolyBase
label_desc: Use image node_id 3 for calling a global function with example usage: _zseries_mul(z1, z2) and returns: np