On the numerical index of real $L_p(\mu)$-spaces

TL;DR

This paper establishes a positive lower bound for the numerical index of real $L_p()$ spaces for $p eq 2$, demonstrating that the index is non-zero and providing explicit inequalities involving bounded linear operators.

Contribution

It provides the first explicit lower bounds for the numerical index of real $L_p()$ spaces, showing it is non-zero for $p eq 2$ and deriving related operator inequalities.

Findings

Numerical index of $L_p()$ is non-zero for $p eq 2$.

Derived explicit lower bounds involving operator norms.

Established inequalities linking the numerical index to integrals of operators.

Abstract

We give a lower bound for the numerical index of the real space $L_p(\mu)$ showing, in particular, that it is non-zero for $p\neq 2$. In other words, it is shown that for every bounded linear operator $T$ on the real space $L_p(\mu)$, one has $$ \sup{\Bigl|\int |x|^{p-1}\sign(x) T x d\mu \Bigr| : x\in L_p(\mu), \|x\|=1} \geq \frac{M_p}{12\e}\|T\| $$ where $M_p=\max_{t\in[0,1]}\frac{|t^{p-1}-t|}{1+t^p}>0$ for every $p\neq 2$. It is also shown that for every bounded linear operator $T$ on the real space $L_p(\mu)$, one has $$ \sup{\int |x|^{p-1}|Tx| d\mu : x\in L_p(\mu), \|x\|=1} \geq \frac{1}{2\e}\|T\|. $$