title: tensor contraction ten1_Air with ten2_Ckr
body: <p>I am trying to contract such tensors ten1_Air with ten2_Ckr, where i = k, to obtain tensor test_ACi</p>
  
  <p>I thought that in the case when i = k, the correct einsum would be:</p>
  
  <pre><code>test_ACi = np.einsum('Air,Cir-&gt;ACi',ten1_Air,ten2_Ckr)
  </code></pre>
  
  <p>However, it seems that the right form is:</p>
  
  <pre><code>test_ACi = np.einsum('Air,Ckr-&gt;ACi',ten1_Air,ten2_Ckr)
  </code></pre>
  
  <p>Which of these contractions is correct and why? I'd be grateful for any explanation. </p>