Invariants of third‐order ordinary differential equations y ′′′ = f ( x , y , y ′, y ′′ ) via point transformations

A new systematic method to find the relative invariant differentiation operators is developed. We incorporate this new approach with Lie's infinitesimal method to study the general class y ′′′ = f ( x , y , y ′, y′′) under general point equivalence transformations in the generic case.As a result, all third‐order differential invariants, relative and absolute invariant differentiation operators are determined for third‐order ODEs y ′′′ = f ( x , y , y ′, y ′′), which are not quadratic in the second‐order derivative. These relative invariant differentiation operators are used to determine the fourth‐order differential invariants and absolute invariant differentiation operators in a degenerate case of interest. As an application, invariant descriptions of all the canonical forms in the complex planewith four infinitesimal symmetries for third‐order ODEs y ′′′ = f ( x , y , y ′, y′′), which are not quadratic in the second‐order derivative, are provided. Copyright © 2015 John Wiley & Sons, Ltd.