LavrenyukS. P., and PtashnikB. I. “On Certain Nonlinear Pseudoparabolic Variational Inequalities Without Initial Conditions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 3, Mar. 1999, pp. 328–337, https://umj.imath.kiev.ua/index.php/umj/article/view/4616.