Extremely non-complex C(K) spaces

TL;DR

This paper demonstrates the existence of infinite-dimensional extremely non-complex Banach spaces, specifically certain $C(K)$ spaces, which satisfy a unique norm equality for all bounded linear operators, answering a previously open question.

Contribution

It constructs examples of extremely non-complex Banach spaces, particularly $C(K)$ spaces with few operators, and answers an open question about their properties.

Findings

Existence of infinite-dimensional extremely non-complex Banach spaces.

Construction of specific $C(K)$ spaces with few operators.

Identification of $C(K)$ spaces containing complemented copies of classical spaces.

Abstract

We show that there exist infinite-dimensional extremely non-complex Banach spaces, i.e. spaces $X$ such that the norm equality $\|Id + T^2\|=1 + \|T^2\|$ holds for every bounded linear operator $T:X\longrightarrow X$. This answers in the positive Question 4.11 of [Kadets, Martin, Meri, Norm equalities for operators, \emph{Indiana U. Math. J.} \textbf{56} (2007), 2385--2411]. More concretely, we show that this is the case of some $C(K)$ spaces with few operators constructed in [Koszmider, Banach spaces of continuous functions with few operators, \emph{Math. Ann.} \textbf{330} (2004), 151--183] and [Plebanek, A construction of a Banach space $C(K)$ with few operators, \emph{Topology Appl.} \textbf{143} (2004), 217--239]. We also construct compact spaces $K_1$ and $K_2$ such that $C(K_1)$ and $C(K_2)$ are extremely non-complex, $C(K_1)$ contains a complemented copy of $C(2^\omega)$ and $C(K_2)$ contains a (1-complemented) isometric copy of $\ell_\infty$.