Dense Banach subalgebras of the null sequence algebra which do not satisfy a differential seminorm condition

TL;DR

This paper constructs specific dense Banach subalgebras of the null sequence algebra that are spectral invariant but do not satisfy a particular differential seminorm condition, highlighting nuanced algebraic properties.

Contribution

It introduces new dense Banach subalgebras of c_0 with spectral invariance that violate the D_1-condition, expanding understanding of algebraic structures without differential seminorms.

Findings

Constructed dense Banach subalgebras of c_0

Subalgebras are spectral invariant

Subalgebras do not satisfy the D_1-condition

Abstract

We construct dense Banach subalgebras $A$ of the null sequence algebra $c_0$ which are spectral invariant, but do not satisfy the $D_1$-condition $\|ab \|_A \leq K(\|a\|_{\infty} \|b\|_A + \|a \|_A \|b \|_{\infty})$, for all $a, b \in A$. The sequences in $A$ vanish in a skewed manner with respect to an unbounded function $\sigma\colon {\mathbb N} \rightarrow [1, \infty)$.