# Stability of average roughness, octahedrality, and strong diameter 2 properties of Banach spaces with respect to absolute sums

## TL;DR

This paper investigates how properties like average roughness, octahedrality, and diameter 2 are preserved or characterized in Banach spaces when forming absolute sums, providing new stability results and optimal bounds.

## Contribution

It establishes how average roughness and diameter 2 properties behave under absolute sums, offering new characterizations and stability results for these geometric properties.

## Key findings

- Absolute sums of $ ext{delta}$-average rough spaces are $ ext{delta}/N(1,1)$-average rough.
- Spaces $X igoplus_p Y$ are $2^{1-1/p}$-average rough for octahedral $X,Y$ and $p
eq 1, ext{infinity}$.
- Diametral strong diameter 2 property is stable only for 1- and $ ext{infinity}$-sums.

## Abstract

We prove that, if Banach spaces $X$ and $Y$ are $\delta$-average rough, then their direct sum with respect to an absolute norm $N$ is $\delta/N(1,1)$-average rough. In particular, for octahedral $X$ and $Y$ and for $p$ in $(1,\infty)$ the space $X\oplus_p Y$ is $2^{1-1/p}$-average rough, which is in general optimal. Another consequence is that for any $\delta$ in $(1,2]$ there is a Banach space which is exactly $\delta$-average rough. We give a complete characterization when an absolute sum of two Banach spaces is octahedral or has the strong diameter 2 property. However, among all of the absolute sums, the diametral strong diameter 2 property is stable only for 1- and $\infty$-sums.