title: How to obtain partial derivative symbol in mathematica
body: <p>I would like Mathematica to return symbolic partial derivative instead of actual derivative.</p>
  
  <pre><code>StressMatrix = ( {
      {\[Sigma]11, \[Sigma]12, \[Sigma]13},
      {\[Sigma]21, \[Sigma]22, \[Sigma]23},
      {\[Sigma]31, \[Sigma]32, \[Sigma]33}
     } );
  varList = ( {
     {\[Rho], \[Theta], z}
    } )
  StressMatrix[[2, 3]]
  varList[[1, 1]]
  D[StressMatrix[[2, 3]], varList[[1, 1]]]
  </code></pre>
  
  <p>The code above returns zero but I would like it to return symbolically Partial derivative of Sigma[[2,3]] with respect to rho.</p>
  
  <p>How can I do this?</p>