e are considering evaluation of
            
              
            
            
               where the variable
               
            
              
            
            
               is sampled from density
               
            
              
            
            .
               We switch the
               density:
            
              
            
            
              
            
            
               We aim to decrease the variance by switching
               from
            
              
            
            
               to
            
              
            
            
               Clearly, the optimal density is
               
            
              
            
            
               The last relationship means that we want to shift the density of our sampling
               to the values of
               
            
              
            
            
               that deliver greater values of
               
            
              
            
            .
            
           
           
           
           
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