Cartan equivalence problem for third order differential operators
  This article is dedicated to solve the equivalence problem for two third order differential operators on the line under general fiber--preserving transformation using the Cartan method of equivalence. We will do three versions of the equivalence problems: first via the direct equivalence problem, second equivalence problem is to determine conditions on two differential operators such that there exists a fiber-preserving transformations mapping one to the other according to gauge equivalence. 
Mehdi Nadjafikhah and Rohollah Bakhshandeh-Chamazkoti
math.DG