Steincy franck Peala, Konan Charles Etienne Goli, and Assohoun Adje. “Existence of Solutions for Nabla-Differentiable Systems of the Second Order”. International Journal of Mathematics And Its Applications, vol. 11, no. 3, Aug. 2023, pp. 71-92, https://ijmaa.in/index.php/ijmaa/article/view/1190.