Types and definable compactness in o-minimality and beyond
Given a theory T, say that a formula φ(x,y) is downward directed if, T⊢∀y1∀y2∃y3∀x(φ(x,y3)→(φ(x,y1)∧φ(x,y2)). We present a theorem stating vaguely that, in an o-minimal theory, any parametrically definable set of formulas that extends to a (complete global) definable type admits a refinement to a downward directed formula. This follows from o-minimal cell decomposition. We explain how this result can be used to prove the equivalence between two known notions of topological compactness for definable topologies in o-minimal structures. We then move on to discuss this theorem in the context of weakly o-minimal and other distal and tame topological settings. In particular, we discuss distal cell decomposition in connection with the theorem.