Barreira, Luis, Jaume Llibre, and Claudia Valls. “Periodic Orbits Near Equilibria via Averaging Theory of Second Order”. Mathematical Modelling and Analysis 17, no. 5 (November 1, 2012): 715-731. Accessed December 2, 2024. https://mma.vgtu.lt/index.php/MMA/article/view/4905.