Bounds for the $p$-angular distance and characterizations of inner product spaces

Abstract

Based on a suitable improvement of a triangle inequality, we derive new mutual bounds for $p$-angular distance ${\alpha }_p[x,y]=\Vert \Vert x{\Vert }^{p-1}x-\Vert y{\Vert }^{p-1}y\Vert$, in a normed linear space $X$. We show that our estimates are more accurate than the previously known upper bounds established by Dragomir, Hile and Maligranda. Next, we give several characterizations of inner product spaces with regard to the $p$-angular distance. In particular, we prove that if $|p|\ge |q|$, $p\ne q$, then $X$ is an inner product space if and only if for every $x,y\in X\setminus \{0\}$, $${\alpha }_p[x,y]\ge \frac{\Vert x{\Vert }^p+\Vert y{\Vert }^p}{\Vert x{\Vert }^q+\Vert y{\Vert }^q}{\alpha }_q[x,y].$$

accepted for publication in Mediterranean Journal of Mathematics