Geometric Aspects of $C^*$-Extreme Points II

Abstract

Building on the results of \cite{HR}, we investigate the relationship between \(C^*\)-extremality and linear extremality in the closed unit balls of general unital \(C^*\)-algebras. We show that, unlike in von Neumann algebras, these two notions of extremality need not coincide in general \(C^*\)-algebras. More precisely, we prove that the unilateral shift is a linear extreme point, but not a \(C^*\)-extreme point, of the closed unit ball of the Toeplitz \(C^*\)-algebra. We further establish that \(C^*\)-extremality implies linear extremality whenever the \(C^*\)-algebra admits a faithful unital representation on a separable Hilbert space. We also prove this implication for elements admitting a factorization $x=v|x|$, where $v$ is a partial isometry belonging to the algebra, and extend the result to suitable quotient \(C^*\)-algebras, including the Calkin algebra. Finally, we characterize the linear extreme points of the closed unit ball of unital \(C^*\)-subalgebras of \(B(H)\) containing the compact operators.