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Periodic solutions of second order nonlinear differential equations. (English) Zbl 0615.34036
The authors study the existence of periodic solutions for second order nonlinear differential equations by using the alternative method for problems at resonance as well as the method of upper and lower solutions. Some previous results are improved.

##### MSC:
 34C25 Periodic solutions to ordinary differential equations 34A34 Nonlinear ordinary differential equations and systems
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##### References:
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