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2. Dulac function for polynomial autonomous systems on a plane;Cherkas;Differ. Equ.,1997
3. A second-degree polynomial Dulac function for a cubic system on the plane;Cherkas;Differ. Equ.,1997
4. A Dulac function in a half-plane in the form of a polynomial of the second degree for a quadratic system;Cherkas;Differ. Equ.,1998
5. Soluzioni periodiche dell'equazione di Liénard generalizatta. Esistenza ed unicità;Conti;Boll. Un. Mat. Ital.,1952