A minimax theorem in infinite-dimensional topological vector spaces

Abstract

In this paper, we obtain a minimax theorem by means of which, in turn, we prove the following result: Let $E$ be an infinite-dimensional reflexive real Banach space, $T:E\to E$ a non-zero compact linear operator, $\varphi:E\to {\bf R}$ a lower semicontinuous, convex and coercive functional, $I\subset {\bf R}$ a compact interval, with $0\in I$, $\psi:I\to {\bf R}$ a lower semicontinuous convex function. Then, for each $r>\varphi(0)$, one has $$\sup_{x\in X}\inf_{\lambda\in I}(\varphi(T(x)-\lambda x)+\psi(\lambda))=r+\psi(0)\ ,$$ where $$X=\{x\in E : \varphi(T(x))\leq r\}\ .$$