1. Sur quelques cas de distinction entre un foyer et un centre;Kukles;Dokl. Akad. Nauk. SSSR,1944
2. On the conditions of Kukles for the existence of a centre;Jin;Bull. London Math. Soc.,1990
3. On the paper of Jin and Wang concerning the conditions for a centre in certain cubic systems;Christopher;Bull. London Math. Soc.,1990
4. Conditions for the equation yy′ = ∑i=03 pi(x)yi to have a centre;Cherkas;Differential Equations,1978
5. On the Kukles and Cherkas center conditions for a cubic system;Shubè;Differential Equations,1993