GalloА., A. M. Piccirillo, and L. Toscano. “On Some Noncoercive Variational Inequalities”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 6, June 2001, pp. 723-41, https://umj.imath.kiev.ua/index.php/umj/article/view/4296.