Messaoudi, S. A., and W. Al-Khulaifi. “General and Optimal Decay for a Viscoelastic Equation With Boundary Feedback”. Topological Methods in Nonlinear Analysis, vol. 51, no. 2, Apr. 2018, pp. 413-27, https://apcz.umk.pl/TMNA/article/view/TMNA.2017.066.