Popovici, N. “Sur Une Notion Abstraite De Quasi-convexité: On an Abstract Notion of Quasi-Convexity”. Rev. Anal. Numér. Théor. Approx., vol. 26, no. 1, Aug. 1997, pp. 191-6, https://ictp.acad.ro/jnaat/journal/article/view/1997-vol26-nos1-2-art25.