A. Aparicio Monforte and J.-A. Weil, A reduction method for higher order variational equations of Hamiltonian systems, In Symmetries and related topics in differential and difference equations, Contemp. Math., Amer. Math. Soc., Providence, RI, 549 (2011), 1-15.doi: 10.1090/conm/549/10850.
T. Combot, Non-integrability of the equal mass; n-body problem with non-zero angular momentum, Celestial Mechanics and Dynamical Astronomy, 114 (2012), 319-340.doi: 10.1007/s10569-012-9417-z.
G. Duval and A. J. Maciejewski, Jordan obstruction to the integrability of Hamiltonian systems with homogeneous potentials, Annales de l'Institut Fourier, 59 (2009), 2839-2890.doi: 10.5802/aif.2510.
G. Duval and A. J. Maciejewski, Integrability of Homogeneous potential of degree $k = \pm 2$. An application of higher variational equations, submited, 2012.
J. J. Morales-Ruiz and J. P. Ramis, A note on the non-integrability of some Hamiltonian systems with a homogeneous potential, Methods Appl. Anal., 8 (2001), 113-120.
E. G. C. Poole, Introduction to the Theory of Linear Differential Equations, Dover Publications Inc., New York, 1960.