A Recursion Operator for the Universal Hierarchy Equation via Cartan's   Method of Equivalence
  We apply Cartan's method of equivalence to find a B\"acklund autotransformation for the tangent covering of the universal hierarchy equation. The transformation provides a recursion operator for symmetries of this equation. 
Oleg I. Morozov
nlin.SI math-ph math.DG math.MP