text: It is sometimes possible to convert an inexact differential into an exact one by means of an integrating factor.
  The most common example of this in thermodynamics is the definition of entropy:
  In this case,  is an inexact differential, because its effect on the state of the system can be compensated by .
  However, when divided by the absolute temperature and when the exchange occurs at reversible conditions (therefore the  subscript), it produces an exact differential: the entropy  is also a state function.