Yang Qigui, Zhang Mou. EXISTENCE OF PERIONDIC SOLUTIONS FOR AUTONOMOUS PLANAR SYSTEMS[J]. Journal of Civil and Environmental Engineering, 1995, 17(1). DOI: 10.11835/j.issn.1674-4764.1995.01.005.
the existence of periodic solutions for autonomous planar systemsis studied and sufficient conditions for the existence of non-zero periodic solutions of this kind areobtained.Some known results are extended and improved.