ORTHOMODULAR LATTICES WHOSE MACNEILLE COMPLETIONS ARE NOT ORTHOMODULAR

The only known example of an orthomodular lattice (abbreviated: OML) whose MacNeille completion is not an OML has been noted independently by several authors, see Adams [1], and is based on a theorem of Ameniya and Araki [2]. This theorem states that for an inner product space V, if we consider the ortholattice L(V, perpendicular-to) = {A subset of V :A = A perpendicular-to perpendicular-to} where A perpendicular-to is the set of elements orthogonal to A, then L(V, perpendicular-to) is an OML if and only if V is complete. Taking the orthomodular lattice L of finite or cofinite dimensional subspaces of an incomplete inner product space V, the ortholattice L(V, perpendicular-to) is a MacNeille completion of L which is not orthomodular. This does not answer the longstanding question Can every OML be embedded into a complete OML? as L can be embedded into the complete OML L(VBAR, perpendicular-to), where VBAR is the completion of the inner product space V. Although the power of the Ameniya-Araki theorem makes the preceding example elegant to present, the ability to picture the situation is lost. In this paper, I present a simpler method to construct OMLs whose MacNeille completions are not orthomodular. No use is made of the Ameniya-Araki theorem. Instead, this method is based on a construction introduced by Kalmbach [7] in which the Boolean algebras generated by the chains of a lattice are glued together to form an OML. A simple method to complete these OMLs is also given. The final section of this paper briefly covers some elementary properties of the Kalmbach construction. I have included this section because I feel that this construction may be quite useful for many purposes and virtually no literature has been written on it.