Integration of systems of two second-order ordinary differential equations with a small parameter that admit four essential operators

We discuss an algorithm for integrating systems of two second-order ordinary differential equations (ODE) with a small parameter that admit approximate Lie algebras with four essential generators. The algorithm is a modification of the method of consecutive order reduction and is based on using operators of invariant differentiation. A special attention is given to the peculiarities of its application in dependence of the structural properties of Lie algebras of approximate symmetries.