Abstract. Many properties of compacta have "textbook" definitions which are phrased in lattice-theoretic terms that, ostensibly, apply only to the full closed-set lattice of a space. We provide a simple criterion for identifying such definitions that may be paraphrased in terms that apply to all lattice bases of the space, thereby making model-theoretic tools available to study the defined properties. In this note we are primarily interested in properties of continua related to unicoherence; i.e., properties that speak to the existence of "holes" in a continuum and in certain of its subcontinua.
the expressibility lemmaWe continue our study [4] of compacta (i.e., compact Hausdorff spaces), especially of continua (i.e., connected compacta), from the perspective of model theory. Many of the classic definitions of properties pertaining to compacta are easily phrased in finitistic lattice-theoretic terms involving closed sets. (It is convenient to use closed-rather than, say, open-set phraseology because, in the setting of compacta, closed sets are compacta themselves.) For any space X, let F (X) be the collection of all closed subsets of X, viewed as a bounded lattice; i.e., as a special structure for the first-order alphabet L = {⊔, ⊓, ⊥, ⊤}. As one would expect, ⊔ is interpreted as union, ⊓ as intersection, ⊥ as ∅, and ⊤ as X. A sublattice A of F (X) is a lattice base for X if every member of F (X) is an intersection of members of A.What makes the notion of lattice base so important in our study of compacta is the following Wallman-style representation theorem (see [11]): There is a sentence in the first-order language L ωω over L, whose models are precisely the lattices that are isomorphic to the lattice bases for compacta.