Year: 2026
Type: preprint
Abstract: A Hermitian completely bounded map $Φ:\mathscr{A}\to B(\mathcal{H})$ need not have a canonical completely positive analogue of an absolute value. We study this problem through the ordered set $\mathsf{E}(Φ)$ of completely positive symmetric majorants of $Φ$, which we call Wittstock envelopes. We show that every Wittstock envelope canonically determines a minimal Wittstock envelope below it. However, minimal envelopes need not be unique. We therefore study the stronger question of when a least completely positive majorant exists. We obtain a criterion, in terms of central supports of spectral projections, for the existence of least majorants below a fixed envelope. In finite dimensions, the Choi representation gives a simple characterization: a least completely positive majorant exists if and only if \(Φ\) or \(-Φ\) is completely positive. Finally, we use the envelope-relative modulus to define envelope-relative perspectives for Hermitian completely bounded maps. These constructions extend Okayasu's completely positive map perspectives, reduce to the usual perspective in the completely positive case, and remain dependent in general on the chosen Wittstock envelopes.
Source: arXiv (Cornell University)
Author: Kian, Mohsen
Language: English
Subfield: Mathematical Physics
Field: Mathematics
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7215702603