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We assume the existence of a supercompact cardinal and produce a model with weak square but no very good scale at a particular cardinal. This follows work of Cummings, Foreman, and Magidor, but uses a different approach. We produce another model, starting from countably many supercompact cardinals, where □K,<K holds but □K, λ fails for λ < K.
We investigate the class of bipartite Borel graphs organized by the order of Borel homomorphism. We show that this class is unbounded by finding a jump operator for Borel graphs analogous to a jump operator of Louveau for Borel equivalence relations. The proof relies on a nonseparation result for iterated Fréchet ideals and filters due to Debs and Saint Raymond. We give a new proof of this fact using effective descriptive set theory. We also investigate an analogue of the Friedman-Stanley jump for Borel graphs. This analogue does not yield a jump operator for bipartite Borel graphs. However, we use it to answer a question of Kechris and Marks by showing that there is a Borel graph with no Borel homomorphism to a locally countable Borel graph, but each of whose connected components has a countable Borel coloring.
We continue our study of the class
${\cal C}\left( D \right)$
, where D is a uniform ultrafilter on a cardinal κ and
${\cal C}\left( D \right)$
is the class of all pairs
$\left( {{\theta _1},{\theta _2}} \right)$
, where
$\left( {{\theta _1},{\theta _2}} \right)$
is the cofinality of a cut in
${J^\kappa }/D$
and J is some
${\left( {{\theta _1} + {\theta _2}} \right)^ + }$
-saturated dense linear order. We give a combinatorial characterization of the class
${\cal C}\left( D \right)$
. We also show that if
$\left( {{\theta _1},{\theta _2}} \right) \in {\cal C}\left( D \right)$
and D is
${\aleph _1}$
-complete or
${\theta _1} + {\theta _2} > {2^\kappa }$
, then
${\theta _1} = {\theta _2}$
.
Let A ≤ B be structures, and
${\cal K}$
a class of structures. An element b ∈ B is dominated by A relative to
${\cal K}$
if for all
${\bf{C}} \in {\cal K}$
and all homomorphisms g, g' : B → C such that g and g' agree on A, we have gb = g'b. Our main theorem states that if
${\cal K}$
is closed under ultraproducts, then A dominates b relative to
${\cal K}$
if and only if there is a partial function F definable by a primitive positive formula in
${\cal K}$
such that FB(a1,…,an) = b for some a1,…,an ∈ A. Applying this result we show that a quasivariety of algebras
${\cal Q}$
with an n-ary near-unanimity term has surjective epimorphisms if and only if
$\mathbb{S}\mathbb{P}_n \mathbb{P}_u \left( {\mathcal{Q}_{{\text{RSI}}} } \right)$
has surjective epimorphisms. It follows that if
${\cal F}$
is a finite set of finite algebras with a common near-unanimity term, then it is decidable whether the (quasi)variety generated by
${\cal F}$
has surjective epimorphisms.
We investigate the property of strict coherence in the setting of many-valued logics. Our main results read as follows: (i) a map from an MV-algebra to [0,1] is strictly coherent if and only if it satisfies Carnap’s regularity condition, and (ii) a [0,1]-valued book on a finite set of many-valued events is strictly coherent if and only if it extends to a faithful state of an MV-algebra that contains them. Remarkably this latter result allows us to relax the rather demanding conditions for the Shimony-Kemeny characterisation of strict coherence put forward in the mid 1950s in this Journal.
In [12], Schwichtenberg showed that the System T definable functionals are closed under a rule-like version Spector’s bar recursion of lowest type levels 0 and 1. More precisely, if the functional Y which controls the stopping condition of Spector’s bar recursor is T-definable, then the corresponding bar recursion of type levels 0 and 1 is already T-definable. Schwichtenberg’s original proof, however, relies on a detour through Tait’s infinitary terms and the correspondence between ordinal recursion for
$\alpha < {\varepsilon _0}$
and primitive recursion over finite types. This detour makes it hard to calculate on given concrete system T input, what the corresponding system T output would look like. In this paper we present an alternative (more direct) proof based on an explicit construction which we prove correct via a suitably defined logical relation. We show through an example how this gives a straightforward mechanism for converting bar recursive definitions into T-definitions under the conditions of Schwichtenberg’s theorem. Finally, with the explicit construction we can also easily state a sharper result: if Y is in the fragment Ti then terms built from
$BR^{\mathbb{N},\sigma } $
for this particular Y are definable in the fragment
${T_{i + {\rm{max}}\left\{ {1,{\rm{level}}\left( \sigma \right)} \right\} + 2}}$
.
We show there exists a complete theory in a language of size continuum possessing a unique atomic model which is not constructible. We also show it is consistent with
$ZFC + {\aleph _1} < {2^{{\aleph _0}}}$
that there is a complete theory in a language of size
${\aleph _1}$
possessing a unique atomic model which is not constructible. Finally we show it is consistent with
$ZFC + {\aleph _1} < {2^{{\aleph _0}}}$
that for every complete theory T in a language of size
${\aleph _1}$
, if T has uncountable atomic models but no constructible models, then T has
${2^{{\aleph _1}}}$
atomic models of size
${\aleph _1}$
.
A Turing degree d is the degree of categoricity of a computable structure
${\cal S}$
if d is the least degree capable of computing isomorphisms among arbitrary computable copies of
${\cal S}$
. A degree d is the strong degree of categoricity of
${\cal S}$
if d is the degree of categoricity of
${\cal S}$
, and there are computable copies
${\cal A}$
and
${\cal B}$
of
${\cal S}$
such that every isomorphism from
${\cal A}$
onto
${\cal B}$
computes d. In this paper, we build a c.e. degree d and a computable rigid structure
${\cal M}$
such that d is the degree of categoricity of
${\cal M}$
, but d is not the strong degree of categoricity of
${\cal M}$
. This solves the open problem of Fokina, Kalimullin, and Miller [13].
For a computable structure
${\cal S}$
, we introduce the notion of the spectral dimension of
${\cal S}$
, which gives a quantitative characteristic of the degree of categoricity of
${\cal S}$
. We prove that for a nonzero natural number N, there is a computable rigid structure
${\cal M}$
such that
$0\prime$
is the degree of categoricity of
${\cal M}$
, and the spectral dimension of
${\cal M}$
is equal to N.
For G a group definable in a saturated model of a NIP theory T, we prove that there is a smallest type-definable subgroup H of G such that the quotient G / H is stable. This generalizes the existence of G00, the smallest type-definable subgroup of G of bounded index.
Answering a special case of a question of Chernikov and Simon, we show that any non-dividing formula over a model M in a distal NIP theory is a member of a consistent definable family, definable over M.
In ZFC, the class Ord of ordinals is easily seen to satisfy the definable version of strong inaccessibility. Here we explore deeper ZFC-verifiable combinatorial properties of Ord, as indicated in Theorems A & B below. Note that Theorem A shows the unexpected result that Ord is never definably weakly compact in any model of ZFC.
Theorem A. Let
${\cal M}$
be any model of ZFC.
(1)The definable tree property fails in
${\cal M}$
: There is an
${\cal M}$
-definable Ord-tree with no
${\cal M}$
-definable cofinal branch.
(2)The definable partition property fails in
${\cal M}$
: There is an
${\cal M}$
-definable 2-coloring
$f:{[X]^2} \to 2$
for some
${\cal M}$
-definable proper class X such that no
${\cal M}$
-definable proper classs is monochromatic for f.
(3)The definable compactness property for
${{\cal L}_{\infty ,\omega }}$
fails in
${\cal M}$
: There is a definable theory
${\rm{\Gamma }}$
in the logic
${{\cal L}_{\infty ,\omega }}$
(in the sense of
${\cal M}$
) of size Ord such that every set-sized subtheory of
${\rm{\Gamma }}$
is satisfiable in
${\cal M}$
, but there is no
${\cal M}$
-definable model of
${\rm{\Gamma }}$
.
Theorem B. The definable ⋄Ordprinciple holds in a model
${\cal M}$
of ZFC iff
${\cal M}$
carries an
${\cal M}$
-definable global well-ordering.
Theorems A and B above can be recast as theorem schemes in ZFC, or as asserting that a single statement in the language of class theory holds in all ‘spartan’ models of GB (Gödel-Bernays class theory); where a spartan model of GB is any structure of the form
$\left( {{\cal M},{D_{\cal M}}} \right)$
, where
${\cal M} \models {\rm{ZF}}$
and
${D_{\cal M}}$
is the family of
${\cal M}$
-definable classes. Theorem C gauges the complexity of the collection GBspa of (Gödel-numbers of) sentences that hold in all spartan models of GB.
We introduce and study the framework of compact metric structures and their associated notions of isomorphisms such as homeomorphic and bi-Lipschitz isomorphism. This is subsequently applied to model various classification problems in analysis such as isomorphism of C*-algebras and affine homeomorphism of Choquet simplices, where among other things we provide a simple proof of the completeness of the isomorphism relation of separable, simple, nuclear C*-algebras recently established by M. Sabok.
We show that if a first-order structure
${\cal M}$
, with universe ℤ, is an expansion of (ℤ,+,0) and a reduct of (ℤ,+,<,0), then
${\cal M}$
must be interdefinable with (ℤ ,+,0) or (ℤ ,+,<,0).
We introduce a forcing technique to construct three-dimensional arrays of generic extensions through FS (finite support) iterations of ccc posets, which we refer to as 3D-coherent systems. We use them to produce models of new constellations in Cichoń’s diagram, in particular, a model where the diagram can be separated into 7 different values. Furthermore, we show that this constellation of 7 values is consistent with the existence of a
${\rm{\Delta }}_3^1$
well-order of the reals.
Downey and Kurtz asked whether every orderable computable group is classically isomorphic to a group with a computable ordering. By an order on a group, one might mean either a left-order or a bi-order. We answer their question for left-orderable groups by showing that there is a computable left-orderable group which is not classically isomorphic to a computable group with a computable left-order. The case of bi-orderable groups is left open.
I analyze the hierarchies of the bounded and the weak bounded forcing axioms, with a focus on their versions for the class of subcomplete forcings, in terms of implications and consistency strengths. For the weak hierarchy, I provide level-by-level equiconsistencies with an appropriate hierarchy of partially remarkable cardinals. I also show that the subcomplete forcing axiom implies Larson’s ordinal reflection principle at ω2, and that its effect on the failure of weak squares is very similar to that of Martin’s Maximum.
I analyze the hierarchies of the bounded resurrection axioms and their “virtual” versions, the virtual bounded resurrection axioms, for several classes of forcings (the emphasis being on the subcomplete forcings). I analyze these axioms in terms of implications and consistency strengths. For the virtual hierarchies, I provide level-by-level equiconsistencies with an appropriate hierarchy of virtual partially super-extendible cardinals. I show that the boldface resurrection axioms for subcomplete or countably closed forcing imply the failure of Todorčević’s square at the appropriate level. I also establish connections between these hierarchies and the hierarchies of bounded and weak bounded forcing axioms.
Fried and Kollár constructed a fully faithful functor from the category of graphs to the category of fields. We give a new construction of such a functor and use it to resolve a longstanding open problem in computable model theory, by showing that for every nontrivial countable structure
${\cal S}$
, there exists a countable field
${\cal F}$
of arbitrary characteristic with the same essential computable-model-theoretic properties as
${\cal S}$
. Along the way, we develop a new “computable category theory”, and prove that our functor and its partially defined inverse (restricted to the categories of countable graphs and countable fields) are computable functors.
Three central combinatorial properties in set theory are the tree property, the approachability property and stationary reflection. We prove the mutual independence of these properties by showing that any of their eight Boolean combinations can be forced to hold at
${\kappa ^{ + + }}$
, assuming that
$\kappa = {\kappa ^{ < \kappa }}$
and there is a weakly compact cardinal above κ.
If in addition κ is supercompact then we can force κ to be
${\aleph _\omega }$
in the extension. The proofs combine the techniques of adding and then destroying a nonreflecting stationary set or a
${\kappa ^{ + + }}$
-Souslin tree, variants of Mitchell’s forcing to obtain the tree property, together with the Prikry-collapse poset for turning a large cardinal into
${\aleph _\omega }$
.