Integrability and solvability of polynomial Li\'enard differential systems
Abstract
We provide the necessary and sufficient conditions of Liouvillian integrability for Li\'enard differential systems describing nonlinear oscillators with a polynomial damping and a polynomial restoring force. We prove that Li\'enard differential systems are not Darboux integrable excluding subfamilies with certain restrictions on the degrees of the polynomials arising in the systems. We demonstrate that if the degree of a polynomial responsible for the restoring force is greater than the degree of a polynomial producing the damping, then a generic Li\'enard differential system is not Liouvillian integrable with the exception of linear Li\'enard systems. However, for any fixed degrees of the polynomials describing the damping and the restoring force we present subfamilies possessing Liouvillian first integrals. As a by-product of our results, we find a number of novel Liouvillian integrable subfamilies. In addition, we study the existence of non-autonomous Darboux first integrals and non-autonomous Jacobi last multipliers with a time-dependent exponential factor.
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