Varieties of Birkhoff Systems Part II

This is the second part of a two-part paper on Birkhoff systems. A Birkhoff system is an algebra that has two binary operations · and +, with each being commutative, associative, and idempotent, and together satisfying x · (x + y) = x + (x · y). The first part of this paper described the lattice of subvarieties of Birkhoff systems. This second part continues the investigation of subvarieties of Birkhoff systems. The 4-element subdirectly irreducible Birkhoff systems are described, and the varieties they generate are placed in the lattice of subvarieties. The poset of varieties generated by finite splitting bichains is described. Finally, a structure theorem is given for one of the five covers of the variety of distributive Birkhoff systems, the only cover that previously had no structure theorem. This structure theorem is used to complete results from the first part of this paper describing the lower part of the lattice of subvarieties of Birkhoff systems.