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2. Periodic solutions of dissipative dynamical systems with singular potentials;Habets;Diff. Integral Eqns,1990
3. Dynamical systems with effective-like potential;Coti Zelati;Nonlinear Analysis,1988
4. Periodic solutions of some Liénard equations with singularities;Habets,1989
5. On periodic solutions of nonlinear differential equations with singularities;Lazer,1987