Mathematical Notes, cilt.115, sa.3-4, ss.626-629, 2024 (SCI-Expanded)
Abstract: We prove that any strictly convex and closed set in is an affine subspace if it contains a hyperplane as a subset. In other words, no hyperplane fits into a strictly convex and closed set unless is flat. We also present certain applications of this result in economic theory reminiscent of the separating and supporting hyperplane theorems.