# Algebraic orthogonality and commuting projections in operator algebras

## TL;DR

This paper develops a framework for non-commutative vector lattices using absolute order unit spaces, introducing concepts like absolute compatibility and order projections, and extends spectral theory to this setting.

## Contribution

It introduces absolute order unit spaces as models for non-commutative vector lattices, defining absolute compatibility and order projections, and extends spectral decomposition theory.

## Key findings

- Absolute compatibility coincides with commutativity for projections in C*-algebras.
- Order projections generalize projections in C*-algebras to order unit spaces.
- Spectral decomposition theory is extended to absolute order unit spaces.

## Abstract

We describe absolutely ordered $p$-normed spaces, for $1 \le p \le \infty$ which presents a model for "non-commutative" vector lattices and includes order theoretic orthogonality. To demonstrate its relevance, we introduce the notion of {\it absolute compatibility} among positive elements in absolute order unit spaces and relate it to symmetrized product in the case of a C$^{\ast}$-algebra. In the latter case, whenever one of the elements is a projection, the elements are absolutely compatible if and only if they commute. We develop an order theoretic prototype of the results. For this purpose, we introduce the notion of {\it order projections} and extend the results related to projections in a unital C$^{\ast}$-algebra to order projections in an absolute order unit space. As an application, we describe spectral decomposition theory for elements of an absolute order unit space.