On the infimum of certain functionals

Abstract

In this note, in particular, we establish the following result: Let $X$ be a real Banach space, $\varphi\in X^*\setminus \{0\}$ and $\psi:X\to {\bf R}$ a Lipschitzian functional with Lipschitz constant equal to $\varphi\|_X^{*}$. Then, we have $$\max\left\{\inf_{x\in X}(\varphi(x)+\psi(x)),\inf_{x\in X}(\varphi(x)-\psi(x))\right\}=\inf_{x\in X}(\varphi(x)+|\psi(x)|)$$ and $$\liminf_{\|x\|\to +\infty}(\varphi(x)+|\psi(x)|)=\inf_{x\in X}(\varphi(x)+|\psi(x)|)\ .$$