A distance formula related to a family of projections orthogonal to their symmetries

Abstract

Let u be a hermitian involution, and e an orthogonal projection, acting on the same Hilbert space. We establish the exact formula, in terms of the norm of eue, for the distance from e to the set of all orthogonal projections q from the algebra generated by e,u, and such that quq=0.

arXiv comment6 pages, 1 figure