UC Berkeley

Let L = (L i {pipe} i ∈ I) be a family of lattices in a nontrivial lattice variety V, and let φ i: L i → M, for i ∈ I, be isotone maps (not assumed to be lattice homomorphisms) to a common lattice M (not assumed to lie in V). We show that the maps φ i can be extended to an isotone map φ:L → M, where L = Free VL is the free product of the L i in V. This was known for V = L, the variety of all lattices. The above free product L can be viewed as the free lattice in V on the partial lattice P formed by the disjoint union of the L i. The analog of the above result does not, however, hold for the free lattice L on an arbitrary partial lattice P. We show that the only codomain lattices M for which that more general statement holds are the complete lattices. On the other hand, we prove the analog of our main result for a class of partial lattices P that are not-quite-disjoint unions of lattices. We also obtain some results similar to our main one, but with the relationship lattices: orders replaced either by semilattices: orders or by lattices: semilattices. Some open questions are noted. © 2012 Springer Basel AG.