Definition of "nondifferentiability"

In this paper we propose a framework that combines the two ideas, and includes two improvements over previously published work on convex-concave programming, specifically the handling of domains of the functions, and the issue of nondifferentiability on the boundary of the domains.

2016, Xinyue Shen, Steven Diamond, Yuantao Gu, Stephen Boyd, “Disciplined Convex-Concave Programming”, in arXiv‎