1. Zhang Pingguang, Cai Suilin, Quadratic systems with a weak focus, Bull. Math. Austral. Math. Soc., 1991, 44: 511–526.
2. Zhang Pingguang, Li Wenhua, A quadratic system with weak focus and strong focus, Ann. Differential Equations, 1992, 8(1): 122–128.
3. Zhang Pingguang, Quadratic systems with a weak focus and a strong focus, Appl. Math. J. Chinese Univ. Ser. B, 1999, 14: 7–14.
4. Li Chengzhi, Non-existence LCs around a fine focus of order 3 of quadratic systems, Chinese Ann. Math. Ser. B, 1986, 7(2): 174–190.
5. Zhang Pingguang, On the uniqueness of the limit cycle around a 2nd-order weak focus for quadratic systems, Acta Math. Sinica, 1999, 42(2): 289–304.