Nurminski, E.A. <https://pure.iiasa.ac.at/view/iiasa/2250.html> ORCID: https://orcid.org/0000-0002-7236-6955 <https://orcid.org/0000-0002-7236-6955>  (1979).  On ε-Subgradient Methods of Non-Differentiable Optimization.    In:  International Symposium on Systems Optimization and Analysis.  pp. 187-195 Germany: Springer Berlin Heidelberg.  ISBN 978-3-662-39921-7 10.1007/978-3-662-39921-7_18 <https://doi.org/10.1007/978-3-662-39921-7_18>.