A companion preorder to G-majorization and a Tarski type fixed-point theorem section: convex analysis

In this paper, a companion preorder ≺Gcomp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \prec _G^\textrm{comp}\,$$\end{document} to G-majorization ≺G\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \prec _G $$\end{document} of Eaton type is introduced and studied. Attention is paid to the case of effective groups G. A criterion for G-majorization inequalities to hold is established by utilizing that companion preorder. A characterization of Gateaux differentiable ≺Gcomp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \prec _G^\textrm{comp}\,$$\end{document}-increasing functions is provided using their gradients. Next, some G-majorization relations are derived for gradients of some functions. New classes of c-strongly (weakly) ≺G\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \prec _G $$\end{document}-increasing functions and c-strongly (weakly) ≺Gcomp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \prec _G^\textrm{comp}\,$$\end{document}-increasing functions are introduced. A Tarski like theorem is established on fixed points of the gradients maps of weakly ≺Gcomp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \prec _G^\textrm{comp}\,$$\end{document}-increasing functions. Some interpretations for the weak-majorization preorder and singular values of matrices are shown.