Ardjouni, A., and A. Djoudi. 2013. “EXISTENCE OF POSITIVE PERIODIC SOLUTIONS FOR A NONLINEAR NEUTRAL DIFFERENCE EQUATION WITH VARIABLE DELAY”. Journal of Nonlinear Analysis and Optimization: Theory & Applications 4 (2):31-38. http://www.math.sci.nu.ac.th/ojs302/index.php/jnao/article/view/280.