sentences:  We take the parameter formula_35 to be the difference between the two times in which the function is calculated.While correlation functions can more easily be described using limits of the strength of the field and limits placed on the time of the system, they can be found more generally as well.For resonance fluorescence, the most important correlation functions are
  
  formula_52,
  
  formula_53,
  
  formula_54,
  
  where
  
  formula_55,
  
  formula_56,
  
  formula_57.