中国科学院数学与系统科学研究院期刊网
ON THE DECISION PROBLEM OF THE CENTRE OR FOCUS FOR THE SYSTEM x=φ(y)-F(x), y=-g(x)
Cai Sui-lin, Zhang Ping-guang
Acta Mathematicae Applicatae Sinica . 1993, (1): 107 -113 .  DOI: 10.12387/C1993013