Published online by Cambridge University Press: 22 March 2023
Suppose that X and Y are two real normed spaces. A map $f:X\rightarrow Y$ is called a min-phase-isometry if it satisfies
$$ \begin{align*} \min\{\|f(x)+f(y)\|,\|f(x)-f(y)\|\}=\min\{\|x+y\|,\|x-y\|\} \quad (x,y\in X). \end{align*} $$
We present properties of min-phase-isometries in the case that Y is strictly convex and show that if a min-phase-isometry f (not necessarily surjective) fixes the origin, then it is phase-equivalent to a linear isometry, that is, $f(x)=\varepsilon (x)g(x)$ for
$x\in X$, where
$g:X\rightarrow Y$ is a linear isometry and
$\varepsilon $ is a map from X to
$\{-1,1\}$.
The first author is supported by the Natural Science Foundation of China (Grant No. 12271402) and the Natural Science Foundation of Tianjin City (Grant No. 22JCYBJC00420).