From: hongyi.zhao@gmail.com   
      
   Haldane gave a talk on his 2012 ICTP Dirac Medal, of which the   
   corresponding presentation can be retrieved online from   
   . On the   
   page 6 of this talk in the above-mentioned file, he wrote the   
   following formula:   
      
   \left|\partial_{\mu} \Psi(\boldsymbol{x})\right\rangle \equiv   
   \sum_{i} \frac{\partial u_{i}(\boldsymbol{x})}{\partial x^{\mu}}|i\rangle   
      
   But I'm confused on the symbols used here. Any more hints/explanations   
   willl be highly appreciated.   
      
   Regards,   
   HY   
      
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