We consider the existence of subharmonic solutions of systems of difference equations with periodic perturbations. The theory of coincidence degree, coupled with some detailed a priori estimates, is applied to show that the perturbed system admits subharmonic solutions near a hyperbolic periodic orbit of the unperturbed system if the perturbation is sufficiently small. © 2002 Elsevier Science (USA). All rights reserved.
Zhang, R., Wei, J., & Wu, J. (2002). Subharmonic solutions of systems of difference equations with periodic perturbations. Part I: Existence. Journal of Mathematical Analysis and Applications, 275(2), 495–511. https://doi.org/10.1016/S0022-247X(02)00637-6