Geodesic
The centre-focus problem for analytical systems of Lienard form in degenerate case
Buletinul Academiei de Ştiinţe a Republicii Moldova. Matematica, no. 2 (2003), pp. 37-50.

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For analytical systems of Lienard form in the case of zero eigenvalues of its linear part is obtained the algebraic criterion of the centre existence, which is analogous to the Cherkas's criterion for systems with imaginary eigenvalues of linear part. We give the solution of centre-focus problem for one class of cubic systems. 
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Le Van Linh; A. P. Sadovskii. The centre-focus problem for analytical systems of Lienard form in degenerate case. Buletinul Academiei de Ştiinţe a Republicii Moldova. Matematica, no. 2 (2003), pp. 37-50. http://geodesic.mathdoc.fr/item/BASM_2003_2_a3/

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