Lavrenyuk, S. P., and B. I. Ptashnik. 1999. “On Certain Nonlinear Pseudoparabolic Variational Inequalities Without Initial Conditions”. Ukrains’kyi Matematychnyi Zhurnal 51 (3): 328–337. https://umj.imath.kiev.ua/index.php/umj/article/view/4616.