title: how to 3-way outer product in numpy?
body: <p>About the numpy.outer<a href="https://docs.scipy.org/doc/numpy/reference/generated/numpy.outer.html" rel="nofollow noreferrer"> [link]</a> . </p>
  
  <p>Given two vectors, <code>a = [a0, a1, ..., aM]</code> and <code>b = [b0, b1, ..., bN]</code>, the outer product will be M*N matrix. </p>
  
  <ol>
  <li><p>But how to implement a 3-array outer product, which means : given
  third vector <code>c = [c0, c1, ..., cP]</code>, how to get the outer product
  between the 3 numpy arrays.</p></li>
  <li><p>and how to get n-way outer product for n-array in numpy, for the method of <code>einsum</code>, how to change <code>'i,j,k-&gt;ijk'</code>  to process <code>"n"</code> . </p></li>
  </ol>