In intuitionistic analysis, Brouwer’s Continuity Principle implies, together with an Axiom of Countable Choice, that the positively Borel sets form a genuinely growing hierarchy: every level of the hierarchy contains sets that do not occur at any lower level.

We give a novel application of algebraic logic to first order logic. A new, flexible construction is presented for representable but not completely representable atomic relation and cylindric algebras of dimension n (for finite n≥2) with the additional property that they are one-generated and the set of all n by n atomic matrices forms a cylindric basis. We use this construction to show that the classical Henkin-Orey omitting types theorem fails for the finite variable fragments of first order logic as long as the number of variables available is ≥2 and we have a binary relation symbol in our language. We also prove a stronger result to the effect that there is no finite upper bound for the extra variables needed in the witness formulas. This result further emphasizes the ongoing interplay between algebraic logic and first order logic.

We describe which subdirectly irreducible flat algebras arise in the variety generated by an arbitrary class of flat algebras with absorbing bottom element. This is used to give an elementary translation of the universal Horn logic of algebras, partial algebras, and more generally still, partial structures into the equational logic of conventional algebras. A number of examples and corollaries follow. For example, the problem of deciding which finite algebras of some fixed type have a finite basis for their quasi-identities is shown to be equivalent to the finite identity basis problem for the finite members of a finitely based variety with definable principal congruences.

The logic ℒ(𝒬_u) extends first-order logic by a generalized form of counting quantifiers (“the number of elements satisfying … belongs to the set C”). This logic is investigated for structures with an injectively ω-automatic presentation. If first-order logic is extended by an infinity-quantifier, the resulting theory of any such structure is known to be decidable [6]. It is shown that, as in the case of automatic structures [21], also modulo-counting quantifiers as well as infinite cardinality quantifiers (“there are κ many elements satisfying …”) lead to decidable theories. For a structure of bounded degree with injective ω-automatic presentation, the fragment of ℒ(𝒬_u) that contains only effective quantifiers is shown to be decidable and an elementary algorithm for this decision is presented. Both assumptions (ω-automaticity and bounded degree) are necessary for this result to hold.

This article is devoted to two different generalizations of projective Boolean algebras: openly generated Boolean algebras and tightly σ-filtered Boolean algebras. We show that for every uncountable regular cardinal κ there are 2^κ pairwise non-isomorphic openly generated Boolean algebras of size κ≥ℵ₁ provided there is an almost free non-free abelian group of size κ. The openly generated Boolean algebras constructed here are almost free. Moreover, for every infinite regular cardinal κ we construct 2^κ pairwise non-isomorphic Boolean algebras of size κ that are tightly σ-filtered and c.c.c. These two results contrast nicely with Koppelberg’s theorem in [12] that for every uncountable regular cardinal κ there are only 2^<κ isomorphism types of projective Boolean algebras of size κ.

We show that any (atomic) excellent class 𝔎 can be expanded with hyperimaginaries to form an (atomic) excellent class 𝔎^eq which has canonical bases. When 𝔎 is, in addition, of finite U-rank, then 𝔎^eq is also simple and has a full canonical bases theorem. This positive situation contrasts starkly with homogeneous model theory for example, where the eq-expansion may fail to be homogeneous. However, this paper shows that expanding an ω-stable, homogeneous class 𝔎 gives rise to an excellent class, which is simple if 𝔎 is of finite U-rank.

We study the relationship between the semistationary reflection principle and stationary reflection principles. We show that for all regular cardinals λ ≥ ω₂ the semistationary reflection principle in the space [λ]^ω implies that every stationary subset of E^λ_ω := { α ∈ λ | cf(α) = ω } reflects. We also show that for all cardinals λ ≥ ω₃ the semistationary reflection principle in [λ]^ω does not imply the stationary reflection principle in [λ]^ω.

For a model ℳ of Peano Arithmetic, let Lt(ℳ) be the lattice of its elementary substructures, and let Lt⁺(ℳ) be the equivalenced lattice (Lt(ℳ), ≅_ℳ), where ≅_ℳ is the equivalence relation of isomorphism on Lt(ℳ). It is known that Lt⁺(ℳ) is always a reasonable equivalenced lattice.

Theorem Let L be a finite distributive lattice and let (L,E) be reasonable. If ℳ₀ is a nonstandard prime model of PA, then ℳ₀ has a cofinal extension ℳ such that Lt⁺(ℳ) ≅ (L,E).

A general method for proving such theorems is developed which, hopefully, will be able to be applied to some nondistributive lattices.

We answer some problems set by Priest in [11] and [12], in particular refuting Priest’s Conjecture that all LP-models of Th(ℕ) essentially arise via congruence relations on classical models of Th(ℕ). We also show that the analogue of Priest’s Conjecture for IΔ₀ + Exp implies the existence of truth definitions for intervals [0,a] ⊂_e M ⊨ IΔ₀ + Exp in any cut [0,a] ⊂_e K ⊆_e M closed under successor and multiplication.

We prove an exponential lower bound on the size of proofs in the proof system operating with ordered binary decision diagrams introduced by Atserias, Kolaitis and Vardi [2]. In fact, the lower bound applies to semantic derivations operating with sets defined by OBDDs. We do not assume any particular format of proofs or ordering of variables, the hard formulas are in CNF. We utilize (somewhat indirectly) feasible interpolation. We define a proof system combining resolution and the OBDD proof system.

We characterize the (κ,λ, <μ)-distributive law in Boolean algebras in terms of cut and choose games 𝔖^κ_<μ(λ), when μ≤κ≤λ and κ^<κ=κ. This builds on previous work to yield game-theoretic characterizations of distributive laws for almost all triples of cardinals κ,λ,μ with μ≤λ, under GCH. In the case when μ≤κ≤λ and κ^<κ=κ, we show that it is necessary to consider whether the κ-stationarity of 𝒫_κ⁺λ in the ground model is preserved by 𝔹. In this vein, we develop the theory of κ-club and κ-stationary subsets of 𝒫_κ⁺λ. We also construct Boolean algebras in which Player I wins 𝔖^κ_κ(κ⁺) but the (κ,∞,κ)-d.l. holds, and, assuming GCH, construct Boolean algebras in which many games are undetermined.

I investigate versions of the Maximality Principles for the classes of forcings which are <κ-closed, <κ-directed-closed, or of the form Col(κ,<λ). These principles come in many variants, depending on the parameters which are allowed. I shall write MP_Γ(A) for the maximality principle for forcings in Γ, with parameters from A. The main results of this paper are:

• The principles have many consequences, such as <κ-closed-generic Σ¹₂(H_κ) absoluteness, and imply, e.g., that ♢_κ holds. I give an application to the automorphism tower problem, showing that there are Souslin trees which are able to realize any equivalence relation, and hence that there are groups whose automorphism tower is highly sensitive to forcing.

• The principles can be separated into a hierarchy which is strict, for many κ.

• Some of the principles can be combined, in the sense that they can hold at many different κ simultaneously.

The possibilities of combining the principles are limited, though: While it is consistent that MP_<κ-closed(H_κ⁺) holds at all regular κ below any fixed α, the “global” maximality principle, stating that MP_<κ-closed(H_κ⁺ ∪ {κ}) holds at every regular κ, is inconsistent. In contrast to this, it is equiconsistent with ZFC that the maximality principle for directed-closed forcings without any parameters holds at every regular cardinal. It is also consistent that every local statement with parameters from H_κ⁺ that’s provably <κ-closed-forceably necessary is true, for all regular κ.

In this paper we show that there is a pair of superhigh r.e. degree that forms a minimal pair. An analysis of the proof shows that a critical ingredient is the growth rates of certain order functions. This leads us to investigate certain high r.e. degrees, which resemble ∅’ very closely in terms of ∅’-jump traceability. In particular, we will construct an ultrahigh degree which is cappable.

We prove new upper bound theorems on the consistency strengths of SPFA(θ), SPFA(θ-linked) and SPFA(θ⁺-cc). Our results are in terms of (θ,Γ)-subcompactness, which is a new large cardinal notion that combines the ideas behind subcompactness and Γ-indescribability. Our upper bound for SPFA(𝔠-linked) has a corresponding lower bound, which is due to Neeman and appears in his follow-up to this paper. As a corollary, SPFA(𝔠-linked) and PFA(𝔠-linked) are each equiconsistent with the existence of a Σ²₁-indescribable cardinal. Our upper bound for SPFA(𝔠-c.c.) is a Σ²₂-indescribable cardinal, which is consistent with V=L. Our upper bound for SPFA(𝔠⁺-linked) is a cardinal κ that is (κ⁺, Σ²₁)-subcompact, which is strictly weaker than κ⁺-supercompact. The axiom MM(𝔠) is a consequence of SPFA(𝔠⁺-linked) by a slight refinement of a theorem of Shelah. Our upper bound for SPFA(𝔠⁺⁺-c.c.) is a cardinal κ that is (κ⁺, Σ²₂)-subcompact, which is also strictly weaker than κ⁺-supercompact.

We define the notion of a weakly pointed tree, and characterize the amount of genericity necessary to prevent a uniformly branching tree being weakly pointed. We use these ideas to show there is no topological analogue of a measure-theoretic selection theorem of Graf and Mauldin.