Vaught's Conjecture states that if Σ is a complete first order theory in a countable language such that Σ has uncountably many pairwise non-isomorphic countably infinite models, then Σ has 2 ℵ 0 many pairwise non-isomorphic countably infinite models.Continuing investigations initiated in [17], we apply methods of algebraic logic to study some variants of Vaught's conjecture. More concretely, let S ⊆ ω ω be a σ-compact monoid. We prove, among other things, that if a complete first order theory Σ has at least ℵ 1 many countable models which cannot be elementarily embedded into each other by elements of S, then, in fact, Σ has continuum many such models. We also study related questions in the context of equality free logics and obtain similar results.Our proofs are based on the representation theory of cylindric and quasipolyadic algebras (for details see [9] and [10]) and topological properties of the Stone spaces of these algebras.

We consider the following dichotomy for Σ 0 2 finitary relations R on analytic subsets of the generalized Baire space for κ: either all R-independent sets are of size at most κ, or there is a κ-perfect Rindependent set. This dichotomy is the uncountable version of a result found in (W. Kubiś, Proc. Amer. Math. Soc. 131 (2003), no 2.:619-623) and in (S. Shelah, Fund. Math. 159 (1999), no. 1:1-50). We prove that the above statement holds assuming ♦ κ and the set theoretical hypothesis I − (κ), which is the modification of the hypothesis I(κ) suitable for limit cardinals. When κ is inaccessible, or when R is a closed binary relation, the assumption ♦ κ is not needed.We obtain as a corollary the uncountable version of a result by G. Sági and the first author (Log. J. IGPL 20 (2012), no. 6:1064-1082) about the κ-sized models of a Σ 1 1 (L κ + κ )-sentence when considered up to isomorphism, or elementary embeddability, by elements of a K κ subset of κ κ. The role of elementary embeddings can be replaced by a more general notion that also includes embeddings, as well as the maps preserving L λµ for ω ≤ µ ≤ λ ≤ κ and the finite variable fragments of these logics.

The open graph dichotomy for a subset X of the Baire space ω ω states that any open graph on X either admits a coloring in countably many colors or contains a perfect complete subgraph. It is a strong version of the open coloring axiom for X that was introduced by Todorčević and Feng to study definable sets of reals. We first show that its recent infinite dimensional generalization by Carroy, Miller and Soukup holds for all subsets of the Baire space in Solovay's model, extending a theorem of Feng from dimension 2. Our main theorem lifts this result to generalized Baire spaces κ κ in two ways.(1) For any regular infinite cardinal κ, the following holds after a Lévy collapse of an inaccessible cardinal λ > κ to κ + . Suppose that H is a κ-dimensional box-open directed hypergraph on a subset of κ κ such that H is definable from a κ-sequence of ordinals. Then either H admits a coloring in κ many colors or there exists a continuous homomorphism from a canonical large directed hypergraph to H. (2) If λ is a Mahlo cardinal, then the previous extends to all relatively box-open directed hypergraphs on any subset of κ κ that is definable from a κ-sequence of ordinals. We derive several applications to definable subsets of generalized Baire spaces, among them variants of the Hurewicz dichotomy that characterizes subsets of Kσ sets, an asymmetric version of the Baire property, an analogue of the Kechris-Louveau-Woodin dichotomy that characterizes when two disjoint sets can be separated by an Fσ set, the determinacy of Väänänen's perfect set game for all subsets of κ κ, and an analogue of the Jayne-Rogers theorem that characterizes the functions which are σ-continuous with closed pieces. Some of these applications lift results of Carroy, Miller and Soukup from the countable setting and extend results of Väänänen, Lücke, Motto Ros and the authors in the uncountable setting.

No abstract