Kasyanov, P. O., and V. S. Mel’nik. “On Properties of Subdifferential Mappings in Fréchet Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 10, Oct. 2005, pp. 1385–1394, https://umj.imath.kiev.ua/index.php/umj/article/view/3692.