Differential invariants of feedback transformations for quasi-harmonic   oscillation equations
  The goal and the main result of the paper is to provide a complete description of the field of rational differential invariants of one class of second order ordinary differential equations with scalar control parameter with respect to Lie pseudo-group of local feedback transformations. In particular, considered class describes behavior of conservative mechanical systems. We construct the class of rational differential invariants that separate regular orbits. It is well known that differential invariants form algebra with respect to the operation of addition and multiplication. In our case, constructed rational differential operators form a field (in algebraic sense). 
D. S. Gritsenko, O. M. Kiriukhin
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