ABSTRACT. A ring with identity is said to be clean if every element can be written as a sum of a unit and an idempotent. The study of clean rings has been at the forefront of ring theory over the past decade. The theory of partially-ordered groups has a nice and long history and since there are several ways of relating a ring to a (unital) partially-ordered group it became apparent that there ought to be a notion of a clean partially-ordered group. In this article we define a clean unital lattice-ordered group; we state and prove a theorem which characterizes clean unital -groups. We mention the relationship of clean unital -groups to algebraic K-theory. In the last section of the article we generalize the notion of clean to the non-unital context and investigate this concept within the framework of W-objects, that is, archimedean -groups with distinguished weak order unit.

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Abstract. All -groups shall be abelian. An a-extension of an -group is an extension preserving the lattice of ideals; an -group with no proper a-extension is called a-closed. A hyperarchimedean -group is one for which each quotient is archimedean. This paper examines hyperarchimedean -groups with unit and their a-extensions by means of the Yosida representation, focussing on several previously open problems.