Geodesic
Rational bases of $GL(2,\mathbb R)$-comitants and of $GL(2,\mathbb R)$-invariants for the planar system of differential equations with nonlinearities of the fourth degree
Buletinul Academiei de Ştiinţe a Republicii Moldova. Matematica, no. 3 (2015), pp. 14-34 
Stanislav Ciubotaru. Rational bases of $GL(2,\mathbb R)$-comitants and of $GL(2,\mathbb R)$-invariants for the planar system of differential equations with nonlinearities of the fourth degree. Buletinul Academiei de Ştiinţe a Republicii Moldova. Matematica, no. 3 (2015), pp. 14-34. http://geodesic.mathdoc.fr/item/BASM_2015_3_a1/
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     title = {Rational bases of $GL(2,\mathbb R)$-comitants and of $GL(2,\mathbb R)$-invariants for the planar system of differential equations with nonlinearities of the fourth degree},
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Voir la notice de l'article provenant de la source Math-Net.Ru

This paper is devoted to the construction of minimal rational bases of $GL(2,\mathbb R)$-comitants and minimal rational bases of $GL(2,\mathbb R)$-invariants for the bidimensional system of differential equations with nonlinearities of the fourth degree. For this system, three minimal rational bases of $GL(2,\mathbb R)$-comitants and two minimal rational bases of $GL(2,\mathbb R)$-invariants were constructed. It was established that any minimal rational basis of $GL(2,\mathbb R)$-comitants contains 13 comitants and each minimal rational basis of $GL(2,\mathbb R)$-invariants contains 11 invariants.

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