“…In Section 6, we study the exceptional case a = −k and conclude that the MoralesRamis method yields no obstruction to integrability, whereas dynamical analysis seems to indicate that the system is not completely integrable. The Morales-Ramis theory was applied to study the integrability of many Hamiltonian systems, see examples in book [17] and in papers [19,20,21,22,23,24,25,26,27,28]. The differential Galois approach was used also for proving non-integrability of non-Hamiltonian systems, see [29,30,31].…”