Indestructible strong compactness and level by level inequivalence

If are such that δ is indestructibly supercompact and γ is measurable, then it must be the case that level by level inequivalence between strong compactness and supercompactness fails. We prove a theorem which points to this result being best possible. Specifically, we show that relative to the existence of cardinals such that κ₁ is λ‐supercompact and λ is inaccessible, there is a model for level by level inequivalence between strong compactness and supercompactness containing a supercompact cardinal in which κ’s strong compactness, but not supercompactness, is indestructible under κ‐directed closed forcing. In this model, κ is the least strongly compact cardinal, and no cardinal is supercompact up to an inaccessible cardinal.     

A note on tall cardinals and level by level equivalence

Starting from a model “κ is supercompact” + “No cardinal is supercompact up to a measurable cardinal”, we force and construct a model such that “κ is supercompact” + “No cardinal is supercompact up to a measurable cardinal” + “δ is measurable iff δ is tall” in which level by level equivalence between strong compactness and supercompactness holds. This extends and generalizes both [ 4 , Theorem 1] and the results of 5 .     

Some properties of infinite factorials

With the Axiom of Choice , for any infinite cardinal but, without , we cannot conclude any relationship between and for an arbitrary infinite cardinal . In this paper, we give some properties of in the absence of and compare them to those of for an infinite cardinal . Among our results, we show that “ for any infinite cardinal and any natural number n” is provable in although “ for any infinite cardinal ” is not.     

The Hypothesis and a supercompact cardinal

In this paper, we prove that: if κ is supercompact and the Hypothesis holds, then there is a proper class of regular cardinals in which are measurable in . Woodin also proved this result independently 11 . As a corollary, we prove Woodin's Local Universality Theorem. This work shows that under the assumption of the Hypothesis and supercompact cardinals, large cardinals in are reflected to be large cardinals in in a local way, and reveals the huge difference between ‐supercompact cardinals and supercompact cardinals under the Hypothesis.     

On the strength of a weak variant of the axiom of counting

In this paper is used to denote Jensen's modification of Quine's ‘new foundations’ set theory () fortified with a type‐level pairing function but without the axiom of choice. The axiom is the variant of the axiom of counting which asserts that no finite set is smaller than its own set of singletons. This paper shows that proves the consistency of the simple theory of types with infinity (). This result implies that proves that consistency of , and that proves the consistency of .     

Free sets for set‐mappings relative to a family of sets

Given a family of subsets of , we try to compute the least natural number n such that for every function there exists a bijection such that for all .     

Building prime models in fully good abstract elementary classes

We show how to build prime models in classes of saturated models of abstract elementary classes (AECs) having a well‐behaved independence relation: Let be an almost fully good AEC that is categorical in and has the ‐existence property for domination triples. For any , the class of Galois saturated models of of size λ has prime models over every set of the form . This generalizes an argument of Shelah, who proved the result when λ is a successor cardinal.     

Clubs on quasi measurable cardinals

We construct a model satisfying “κ is quasi measurable”. Here, we call κ quasi measurable if there is an ℵ₁‐saturated κ‐additive ideal on κ. We also show that, in this model, forcing with adds one but not κ Cohen reals. We introduce a weak club principle and use it to show that, consistently, for some ℵ₁‐saturated κ‐additive ideal on κ, forcing with adds one but not κ random reals.     

The length of an intersection

A poset is well‐partially ordered (WPO) if all its linear extensions are well orders; the supremum of ordered types of these linear extensions is the length , of p . We prove that if the vertex set X is infinite, of cardinality κ, and the ordering ⩽ is the intersection of finitely many well partial orderings of X , , then, letting , with , denote the euclidian division by κ (seen as an initial ordinal) of the length of each corresponding poset: where denotes the least initial ordinal greater than the ordinal . This inequality is optimal. This result answers questions of Forster.     

The tree property and the continuum function below

We say that a regular cardinal κ, , has the tree property if there are no κ‐Aronszajn trees; we say that κ has the weak tree property if there are no special κ‐Aronszajn trees. Starting with infinitely many weakly compact cardinals, we show that the tree property at every even cardinal , , is consistent with an arbitrary continuum function below which satisfies , . Next, starting with infinitely many Mahlo cardinals, we show that the weak tree property at every cardinal , , is consistent with an arbitrary continuum function below which satisfies , . Thus the tree property has no provable effect on the continuum function below except for the trivial requirement that the tree property at implies for every infinite κ.     

A wild model of linear arithmetic and discretely ordered modules

Linear arithmetics are extensions of Presburger arithmetic () by one or more unary functions, each intended as multiplication by a fixed element (scalar), and containing the full induction schemes for their respective languages. In this paper, we construct a model of the 2‐linear arithmetic (linear arithmetic with two scalars) in which an infinitely long initial segment of “Peano multiplication” on is ‐definable. This shows, in particular, that is not model complete in contrast to theories and that are known to satisfy quantifier elimination up to disjunctions of primitive positive formulas. As an application, we show that , as a discretely ordered module over the discretely ordered ring generated by the two scalars, does not have the NIP, answering negatively a question of Chernikov and Hils.     

Decreasing sentences in Simple Type Theory

We present various results regarding the decidability of certain sets of sentences by Simple Type Theory (). First, we introduce the notion of decreasing sentence, and prove that the set of decreasing sentences is undecidable by Simple Type Theory with infinitely many zero‐type elements (); a result that follows directly from the fact that every sentence is equivalent to a decreasing sentence. We then establish two different positive decidability results for a weak subtheory of . Namely, the decidability of (a subset of Σ₁) and (the set of all sentences , where φ is strictly decreasing). Finally, we present some consequences for the set of existential‐universal sentences. All the above results have direct implications for Quine's theory of “New Foundations” () and its weak subtheory .     

The strong tree property and weak square

We show that it is consistent, relative to ω many supercompact cardinals, that the super tree property holds at for all but there are weak square and a very good scale at .     

A note on the deductive strength of the Nielsen‐Schreier theorem

We show that the Boolean Prime Ideal Theorem () does not imply the Nielsen‐Schreier Theorem () in , thus strengthening the result of Kleppmann from “Nielsen‐Schreier and the Axiom of Choice” that the (strictly weaker than ) Ordering Principle () does not imply in . We also show that is false in Mostowski's Linearly Ordered Model of . The above two results also settle the corresponding open problems from Howard and Rubin's “Consequences of the Axiom of Choice”.     

Two kinds of fixed point theorems and reverse mathematics

In this paper, we investigate the logical strength of two types of fixed point theorems in the context of reverse mathematics. One is concerned with extensions of the Banach contraction principle. Among theorems in this type, we mainly show that the Caristi fixed point theorem is equivalent to over . The other is dedicated to topological fixed point theorems such as the Brouwer fixed point theorem. We introduce some variants of the Fan‐Browder fixed point theorem and the Kakutani fixed point theorem, which we call and , respectively. Then we show that is equivalent to and is equivalent to , over . In addition, we also study the application of the Fan‐Browder fixed point theorem to game systems.     

Shelah's eventual categoricity conjecture in tame abstract elementary classes with primes

A new case of Shelah's eventual categoricity conjecture is established:

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Disassociated indiscernibles

This work addresses a basic question by Kunen: how many normal measures can there be on the least measurable cardinal? Starting with a measurable cardinal κ of Mitchell order less than two () we define a Prikry type forcing which turns the number of normal measures over κ to any while making κ the first measurable.     

Two new equivalents of Lindelöf metric spaces

In the realm of Lindelöf metric spaces the following results are obtained in : (i) If is a Lindelöf metric space then it is both densely Lindelöf and almost Lindelöf. In addition, under the countable axiom of choice , the three notions coincide. (ii) The statement “every separable metric space is almost Lindelöf” implies that every infinite subset of has a countably infinite subset). (iii) The statement “every almost Lindelöf metric space is quasi totally bounded implies . (iv) The proposition “every quasi totally bounded metric space is separable” lies, in the deductive hierarchy of choice principles, strictly between the countable union theorem and . Likewise, the statement “every pre‐Lindelöf (or Lindelöf) metric space is separable” lies strictly between and .     

Linear extension operators for continuous functions on definable sets in the p‐adic context

Let E be a subset of . A linear extension operator is a linear map that sends a function on E to its extension on some superset of E . In this paper, we show that if E is a semi‐algebraic or subanalytic subset of , then there is a linear extension operator such that is semi‐algebraic (subanalytic) whenever f is semi‐algebraic (subanalytic).     

Three‐space type Hahn‐Banach properties

In set theory without the axiom of choice , three‐space type results for the Hahn‐Banach property are provided. We deduce that for every Hausdorff compact scattered space K , the Banach space C(K ) of real continuous functions on K satisfies the (multiple) continuous Hahn‐Banach property in . We also prove in Rudin's theorem: “Radon measures on Hausdorff compact scattered spaces are discrete”.