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2. On the number of limit cycles which appear with the variation of coef-ficients from an equilibrium position of focus or center type;Bautin;Mat. Sb. (N.S.),1952
3. On the number of limit cycles of the equation dydy = P(x, y)Q(x, y) where P and Q are polynomials of the second degree;Petrovskii;Mat. Sb.,1955