arXiv subjects
Publications and source records attributed to Ori Segel.
In "VC density in some theories without the independence property" the authors asked whether any partial order of finite width has the VC1 property (i.e. every formula in one variable has UDTFS in one parameter). We give a negative answer and some related remarks.
We study type spaces and saturation for local positive logic.
We develop the basic model theory of local positive logic, a new logic that mixes positive logic (where negation is not allowed) and local logic (where models omit types of infinite distant pairs). We study several basic model theoretic notions such as compactness, positive closedness (existential closedness) and completeness (irreducibility).
We reformulate Hrushovski's definability patterns from the setting of first order logic to the setting of positive logic. Given an h-universal theory T we put two structures on the type spaces of models of T in two languages, \mathcal{L} and \mathcal{L}_{\pi}. It turns out that for sufficiently saturated models, the corresponding h-universal theories \mathcal{T} and \mathcal{T}_{\pi} are independent of the model. We show that there is a canonical model \mathcal{J} of \mathcal{T}, and in many interesting cases there is an analogous canonical model \mathcal{J}_{\pi} of \mathcal{T}_{\pi}, both of which embed into every type space. We discuss the properties of these canonical models, called cores, and give some concrete examples.
The notion of a complete type can be generalized in a natural manner to allow assigning a value in an arbitrary Boolean algebra B to each formula. We show some basic results regarding the effect of the properties of B on the behavior of such types, and show they are particularity well behaved in the case of NIP theories. In particular, we generalize the third author's result about counting types, as well as the notion of a smooth type and extending a type to a smooth one. We then show that Keisler measures are tied to certain Boolean types and show that some of the results can thus be transferred to measures - in particular, giving an alternative proof of the fact that every measure in a dependent theory can be extended to a smooth one. We also study the stable case. We consider this paper as an invitation for more research into the topic of Boolean types.