Non-integrability of the generalized spring-pendulum problem

We investigate a generalization of the three-dimensional spring-pendulum system. The problem depends on two real parameters (k, a), where k is the Young modulus of the spring and a describes the nonlinearity of elastic forces. We show that this system is not integrable when k not equal -a. We carefully investigated the case k = -a when the necessary condition for integrability given by the Morales-Ruiz-Ramis theory is satisfied. We discuss an application of the higher order variational equations for proving the non-integrability in this case.