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## Archive of Issues

Russia Nizhni Novgorod
Year
2019
Volume
29
Issue
4
Pages
483-500
 Section Mathematics Title Application of extreme sub- and epiarguments, convex and concave envelopes to search for global extrema Author(-s) Galkin O.E.a, Galkina S.Yu.a Affiliations National Research University Higher School of Economics, Nizhni Novgoroda Abstract For real-valued functions $f$, defined on subsets of real linear spaces, the notions of extreme subarguments, extreme epiarguments, natural convex $\check{f}$ and natural concave $\hat{f}$ envelopes are introduced. It is shown that for any strictly convex function $g$, any point of the global maximum of the function $f+g$ is an extreme subargument for the function $f$. A similar result is obtained for functions of the form $f/v + g$. Based on these results, a method is proposed, that facilitates the search for global extrema of functions in some cases. It is proved that under certain conditions the functions $f/v+g$ and $\hat{f}/v+g$ have the same global maximum and the same points of the global maximum. Necessary and sufficient conditions for the naturalness of the convex envelope of function are given. A sufficient condition for the invariance of values of the concave envelope $\hat{f}$ during narrowing the domain of $f$ is established. Extreme sub- and epiarguments for continuous nowhere differentiable Gray-Takagi function $K(x)$ of Kobayashi on the segment $[0;1]$ are found. Moreover, the global extrema of the function $K(x)/\cos{x}$ and the global maximum of the function $K(x)-\sqrt{x(1-x)}$ on $[0;1]$ are calculated. The article is provided with examples and graphic illustrations. Keywords nondifferentiable optimization, extreme subarguments (subabscissae) and epiarguments (epiabscissae) of function, natural convex and concave envelopes of function, Gray Takagi function of Kobayashi UDC 517.518.244, 519.6 MSC 26A27, 26A30, 26B25, 49M30, 90C26 DOI 10.20537/vm190402 Received 16 September 2019 Language Russian Citation Galkin O.E., Galkina S.Yu. 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