Geodesic
Solution of the center problem for cubic systems with a bundle of three invariant straight lines
Buletinul Academiei de Ştiinţe a Republicii Moldova. Matematica, no. 1 (2003), pp. 91-101 
Alexandru Şubă. Solution of the center problem for cubic systems with a bundle of three invariant straight lines. Buletinul Academiei de Ştiinţe a Republicii Moldova. Matematica, no. 1 (2003), pp. 91-101. http://geodesic.mathdoc.fr/item/BASM_2003_1_a9/
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Voir la notice de l'article provenant de la source Math-Net.Ru

For cubic differential system with three invariant straight lines which pass through the same point it is proved that a singular point with purely imaginary eigenvalues (weak focus) is a center if and only if the focal values $g_{2j+1}$, $j=\overline{1,5}$, vanish.

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