Dedekind Complete Posets from Sheaves on von Neumann Algebras

摘要

We show that for any two von Neumann algebras M and N, the space of non-unital normal homomorphisms N -> M with finite support, modulo conjugation by unitaries in M, is Dedekind complete with respect to the partial order coming from the addition of homomorphisms with orthogonal ranges; this ties in with work by Brown and Capraro, where the corresponding objects are given Banach space structures under various niceness conditions on M and N. More generally, we associate to M a Grothendieck site-type category, and show that presheaves satisfying a sheaf-like condition on this category give rise to Dedekind complete lattices upon modding out unitary conjugation. Examples include the above-mentioned spaces of morphisms, as well as analogous spaces of completely positive or contractive maps. We also study conditions under which these posets can be endowed with cone structures extending their partial additions.