Inner Models and Large Cardinals

1995 ◽  
Vol 1 (4) ◽  
pp. 393-407 ◽  
Author(s):  
Ronald Jensen
Keyword(s):  
Set Theory ◽  
Cardinal Number ◽  
Ordinal Number ◽  
Infinite Cardinal ◽  
Recursive Definition ◽  
Von Neumann ◽  
Ordinal Numbers ◽  
Kurt Gödel ◽  
Uncountable Cardinal ◽  
Image Position

In this paper, we sketch the development of two important themes of modern set theory, both of which can be regarded as growing out of work of Kurt Gödel. We begin with a review of some basic concepts and conventions of set theory. §0. The ordinal numbers were Georg Cantor's deepest contribution to mathematics. After the natural numbers 0, 1, …, n, … comes the first infinite ordinal number ω, followed by ω + 1, ω + 2, …, ω + ω, … and so forth. ω is the first limit ordinal as it is neither 0 nor a successor ordinal. We follow the von Neumann convention, according to which each ordinal number α is identified with the set {ν ∣ ν α} of its predecessors. The ∈ relation on ordinals thus coincides with <. We have 0 = ∅ and α + 1 = α ∪ {α}. According to the usual set-theoretic conventions, ω is identified with the first infinite cardinal ℵ0, similarly for the first uncountable ordinal number ω1 and the first uncountable cardinal number ℵ1, etc. We thus arrive at the following picture: The von Neumann hierarchy divides the class V of all sets into a hierarchy of sets Vα indexed by the ordinal numbers. The recursive definition reads: (where } is the power set of x); Vλ = ∪v<λVv for limit ordinals λ. We can represent this hierarchy by the following picture.

Martin's axiom and the continuum

1995 ◽  
Vol 60 (2) ◽  
pp. 374-391 ◽  
Author(s):  
Haim Judah ◽  
Andrzej Rosłanowski
Keyword(s):  
Set Theory ◽  
Continuum Hypothesis ◽  
Point Of View ◽  
Georg Cantor ◽  
Mathematical Research ◽  
Kurt Gödel ◽  
Uncountable Cardinal ◽  
Image Position ◽  
Infinite Set ◽  
The Continuum

Since Georg Cantor discovered set theory the main problem in this area of mathematical research has been to discover what is the size of the continuum. The continuum hypothesis (CH) says that every infinite set of reals either has the same cardinality as the set of all reals or has the cardinality of the set of natural numbers, namelyIn 1939 Kurt Gödel discovered the Constructible Universe and proved that CH holds in it. In the early sixties Paul Cohen proved that every universe of set theory can be extended to a bigger universe of set theory where CH fails. Moreover, given any reasonable cardinal κ, it is possible to build a model where the continuum size is κ. The new technique discovered by Cohen is called forcing and is being used successfully in other branches of mathematics (analysis, algebra, graph theory, etc.).In the light of these two stupendous works the experts (especially the platonists) were forced to conclude that from the point of view of the classical axiomatization of set theory (called ZFC) it is impossible to give any answer to the continuum size problem: everything is possible!In private communications Gödel suggested that the continuum size from a platonistic point of view should be ω2, the second uncountable cardinal. As this is not provable in ZFC, Gödel suggested that a new axiom should be added to ZFC to decide that the cardinality of the continuum is ω2.

On a set theory of bernays

1967 ◽  
Vol 32 (3) ◽  
pp. 319-321 ◽  
Author(s):  
Leslie H. Tharp
Keyword(s):  
Set Theory ◽  
Free Variable ◽  
Reflection Principle ◽  
Formal Definition ◽  
Von Neumann ◽  
Starting Point ◽  
Definition Of ◽  
Image Position

We are concerned here with the set theory given in [1], which we call BL (Bernays-Levy). This theory can be given an elegant syntactical presentation which allows most of the usual axioms to be deduced from the reflection principle. However, it is more convenient here to take the usual Von Neumann-Bernays set theory [3] as a starting point, and to regard BL as arising from the addition of the schema where S is the formal definition of satisfaction (with respect to models which are sets) and ┌φ┐ is the Gödel number of φ which has a single free variable X.

Type-raising operations on cardinal and ordinal numbers in Quine's “New foundations”

1973 ◽  
Vol 38 (1) ◽  
pp. 59-68 ◽  
Author(s):  
C. Ward Henson
Keyword(s):  
Set Theory ◽  
Initial Segment ◽  
Initial Section ◽  
Ordinal Numbers ◽  
Cardinal Numbers ◽  
Basic Facts ◽  
Image Position ◽  
The Axiom Of Choice ◽  
Increasing Function ◽  
Standard Operations

In this paper we develop certain methods of proof in Quine's set theory NF which have no counterparts elsewhere. These ideas were first used by Specker [5] in his disproof of the Axiom of Choice in NF. They depend on the properties of two related operations, T(n) on cardinal numbers and U(α) on ordinal numbers, which are defined by the equationsfor each set x and well ordering R. (Here and below we use Rosser's notation [3].) The definitions insure that the formulas T(x) = y and U(x) = y are stratified when y is assigned a type one higher than x. The importance of T and U stems from the following facts: (i) each of T and U is a 1-1, order preserving operation from its domain onto a proper initial section of its domain; (ii) Tand U commute with most of the standard operations on cardinal and ordinal numbers.These basic facts are discussed in §1. In §2 we prove in NF that the exponential function 2n is not 1-1. Indeed, there exist cardinal numbers m and n which satisfyIn §3 we prove the following technical result, which has many important applications. Suppose f is an increasing function from an initial segment S of the set NO of ordinal numbers into NO and that f commutes with U.

On large cardinals and partition relations

1971 ◽  
Vol 36 (2) ◽  
pp. 305-308 ◽  
Author(s):  
E. M. Kleinberg ◽  
R. A. Shore
Keyword(s):  
Set Theory ◽  
Measurable Cardinal ◽  
Order Type ◽  
Large Cardinals ◽  
Infinite Cardinal ◽  
Partition Relation ◽  
Weakly Compact ◽  
Original Result ◽  
Partition Relations ◽  
Uncountable Cardinal

A significant portion of the study of large cardinals in set theory centers around the concept of “partition relation”. To best capture the basic idea here, we introduce the following notation: for x and y sets, κ an infinite cardinal, and γ an ordinal less than κ, we let [x]γ denote the collection of subsets of x of order-type γ and abbreviate with the partition relation for each function F frominto y there exists a subset C of κ of cardinality κ such that (such that for each α < γ) the range of F on [С]γ ([С]α) has cardinality 1. Now although each infinite cardinal κ satisfies the relation for each n and m in ω (F. P. Ramsey [8]), a connection with large cardinals arises when one asks, “For which uncountable κ do we have κ → (κ)2?” Indeed, any uncountable cardinal κ which satisfies κ → (κ)2 is strongly inaccessible and weakly compact (see [9]). As another example one can look at the improvements of Scott's original result to the effect that if there exists a measurable cardinal then there exists a nonconstructible set. Indeed, if κ is a measurable cardinal then κ → (κ)< ω, and as Solovay [11] has shown, if there exists a cardinal κ such that κ → (κ)< ω3 (κ → (ℵ1)< ω, even) then there exists a nonconstructible set of integers.

Singular Cardinals and the PCF Theory

1995 ◽  
Vol 1 (4) ◽  
pp. 408-424 ◽  
Author(s):  
Thomas Jech
Keyword(s):  
Set Theory ◽  
Cardinal Number ◽  
Natural Generalization ◽  
Large Cardinals ◽  
Infinite Cardinal ◽  
Quarter Century ◽  
Pcf Theory ◽  
Cardinal Numbers ◽  
Singular Cardinals ◽  
The Rich

§1. Introduction. Among the most remarkable discoveries in set theory in the last quarter century is the rich structure of the arithmetic of singular cardinals, and its deep relationship to large cardinals. The problem of finding a complete set of rules describing the behavior of the continuum function 2ℵα for singular ℵα's, known as the Singular Cardinals Problem, has been attacked by many different techniques, involving forcing, large cardinals, inner models, and various combinatorial methods. The work on the singular cardinals problem has led to many often surprising results, culminating in a beautiful theory of Saharon Shelah called the pcf theory (“pcf” stands for “possible cofinalities”). The most striking result to date states that if 2ℵn < ℵω for every n = 0, 1, 2, …, then 2ℵω < ℵω4. In this paper we present a brief history of the singular cardinals problem, the present knowledge, and an introduction into Shelah's pcf theory. In Sections 2, 3 and 4 we introduce the reader to cardinal arithmetic and to the singular cardinals problems. Sections 5, 6, 7 and 8 describe the main results and methods of the last 25 years and explain the role of large cardinals in the singular cardinals problem. In Section 9 we present an outline of the pcf theory. §2. The arithmetic of cardinal numbers. Cardinal numbers were introduced by Cantor in the late 19th century and problems arising from investigations of rules of arithmetic of cardinal numbers led to the birth of set theory. The operations of addition, multiplication and exponentiation of infinite cardinal numbers are a natural generalization of such operations on integers. Addition and multiplication of infinite cardinals turns out to be simple: when at least one of the numbers κ, λ is infinite then both κ + λ and κ·λ are equal to max {κ, λ}. In contrast with + and ·, exponentiation presents fundamental problems. In the simplest nontrivial case, 2κ represents the cardinal number of the power set P(κ), the set of all subsets of κ. (Here we adopt the usual convention of set theory that the number κ is identified with a set of cardinality κ, namely the set of all ordinal numbers smaller than κ.

Interpretations of set theory and ordinal number theory

1967 ◽  
Vol 32 (2) ◽  
pp. 145-161
Author(s):  
Mariko Yasugi
Keyword(s):  
Number Theory ◽  
Set Theory ◽  
Ordinal Number ◽  
Recursive Relation ◽  
Ordinal Numbers ◽  
Primitive Recursive

In [3], Takeuti developed the theory of ordinal numbers (ON) and constructed a model of Zermelo-Fraenkel set theory (ZF), using the primitive recursive relation ∈ of ordinal numbers. He proved:(1) If A is a ZF-provable formula, then its interpretation A0 in ON is ON-provable;(2) Let B be a sentence of ordinal number theory. Then B is a theorem of ON if and only if the natural translation B* of B in set theory is a theorem of ZF;(3) (V = L)° holds in ON.

An addendum to “The work of Kurt Gödel”

1978 ◽  
Vol 43 (3) ◽  
pp. 613-613 ◽  
Author(s):  
Stephen C. Kleene
Keyword(s):  
Set Theory ◽  
Continuum Hypothesis ◽  
Axiom System ◽  
Von Neumann ◽  
Main Achievement ◽  
Constructible Sets ◽  
Kurt Gödel ◽  
The One ◽  
The Axiom Of Choice ◽  
The Continuum

Gödel has called to my attention that p. 773 is misleading in regard to the discovery of the finite axiomatization and its place in his proof of the consistency of GCH. For the version in [1940], as he says on p. 1, “The system Σ of axioms for set theory which we adopt [a finite one] … is essentially due to P. Bernays …”. However, it is not at all necessary to use a finite axiom system. Gödel considers the more suggestive proof to be the one in [1939], which uses infinitely many axioms.His main achievement regarding the consistency of GCH, he says, really is that he first introduced the concept of constructible sets into set theory defining it as in [1939], proved that the axioms of set theory (including the axiom of choice) hold for it, and conjectured that the continuum hypothesis also will hold. He told these things to von Neumann during his stay at Princeton in 1935. The discovery of the proof of this conjecture On the basis of his definition is not too difficult. Gödel gave the proof (also for GCH) not until three years later because he had fallen ill in the meantime. This proof was using a submodel of the constructible sets in the lowest case countable, similar to the one commonly given today.

Partition properties of M-ultrafilters and ideals

1987 ◽  
Vol 52 (4) ◽  
pp. 897-907
Author(s):  
Joji Takahashi
Keyword(s):  
Set Theory ◽  
Predicate Symbol ◽  
Infinite Cardinal ◽  
List Type ◽  
Weakly Compact ◽  
Unary Predicate ◽  
Transitive Model ◽  
Basic Set ◽  
Uncountable Cardinal ◽  
The Axiom Of Choice

As is well known, the following are equivalent for any uniform ultrafilter U on an uncountable cardinal:(i) U is selective;(ii) U → ;(iii) U → .In §1 of this paper, we consider this result in terms of M-ultrafilters (Definition 1.1), where M is a transitive model of ZFC (Zermelo-Fraenkel set theory with the axiom of choice). We define the partition properties and for M-ultrafilters (Definition 1.3), and characterize those M-ultrafilters that possess these properties (Theorem 1.5) so that the result mentioned at the beginning is subsumed as the special case that M is V, the universe of all sets. It turns out that the two properties have to be handled separately, and that, besides selectivity, we need to formulate additional conditions (Definition 1.4). The extra conditions become superfluous when M = V because they are then trivially satisfied. One of them is nothing new; it is none other than Kunen's iterability-of-ultrapowers condition.In §2, we obtain characterizations of the partition properties I+ → and I+ → (Definition 2.3) of uniform ideals I on an infinite cardinal κ (Theorem 2.6). This is done by applying the main results of §1 to the canonical -ultrafilter in the Boolean-valued model constructed from the completion of the quotient algebra P(κ)/I. They are related to certain known characterizations of weakly compact and of Ramsey cardinals.Our basic set theory is ZFC. In §1, it has to be supplemented by a new unary predicate symbol M and new nonlogical axioms that make M look like a transitive model of ZFC.

On the existence of a saturated solution of the differential equation x′ = f (t, x)

1977 ◽  
Vol 78 (1-2) ◽  
pp. 97-99
Author(s):  
Johann Walter
Keyword(s):  
Differential Equation ◽  
Set Theory ◽  
Saturated Solution ◽  
Open Domain ◽  
Recursive Definition ◽  
Power Set ◽  
Image Position

SynopsisLet (1) x′ = f(t, x) be any differential equation and S0 the set of solutions of (1) with open domain. It is known that for every g ∊ S0 a non-continuable (= saturated) ∊ S0 exists which is an extension of g. Usually is represented in the form is a sequence in S0 defined by some sort of a variant of what is called ‘recursive definition’ in set theory. It will be shown that a functionexists (P(S0) is the power set of S0) such that the above-mentioned variant can be given the form: There exists a sequence in S0 such that

scholarly journals La hipótesis generalizada del continuo (HGC) y su relación con el axioma de elección (AE)

1989 ◽  
Vol 21 (62) ◽  
pp. 55-66
Author(s):  
José Alfredo Amor
Keyword(s):  
Set Theory ◽  
Continuum Hypothesis ◽  
Cardinal Number ◽  
Ordinal Number ◽  
Axiom Of Choice ◽  
Relative Consistency ◽  
Power Set ◽  
The Many ◽  
Definition Of ◽  
The Axiom Of Choice

The so called Generalized Continuum Hypothesis (GCH) is the sentence: "If A is an infinile set whose cardinal number is K and 2K denotes the cardinal number of the set P(A) of subsets of A (the power set of A), and K + denotes the succesor cardinal of K, then 2K = K +". The Continuum Hypothesis (CH) asserts the particular case K = o. It is clear that GCH implies CH. Another equivalent version of GCH, is the sentence: 'Any subset of the set of subsets of a given infinite set is or of cardinality less or equal than the cardinality of the given set, or of the cardinality of all the set of subsets". Gödel in 1939, and Cohen in 1963, settled the relative consistency of the Axiom of Choice (AC) and of its negation not-AC, respectively, with respecllo the Zermelo-Fraenkel set theory (ZF). On the other hand, Gödel in 1939, and Cohen in 1963 settled too, the relative consistency of GCH , CH and of its negations not-GCH, not-CH, respectively, with respect to the Zermelo-Fraenkel set theory with the Axiom of Choice (ZF + AC or ZFC). From these results we know that GCH and AC are undecidable sentences in ZF set theory and indeed, the most famous undecidable sentences in ZF; but, which is the relation between them? From the above results, in the theory ZF + AC is not demonstrated GCH; it is clear then that AC doesn't imply GCH in ZF theory, Bul does GCH implies AC in ZF theory? The answer is yes! or equivalently, there is no model of ZF +GCH + not-AC. A very easy proof can be given if we have an adecuate definition of cardinal number of a set, that doesn't depend of AC but depending from the Regularity Axiom, which asserls that aIl sets have a range, which is an ordinal number associated with its constructive complexity. We define the cardinal number of A, denoted |A|, as foIlows: |A|= { The least ordinal number equipotent with A, if A is well orderable The set of all sets equipotent with A and of minimum range, in other case. It is clear that without AC, may be not ordinal cardinals and all cardinals are ordinal cardinals if all sets are well orderable (AC). Now we formulate: GCH*: For all ordinal cardinal I<, 2K = I< + In the paper is demonstrated that this formulation GCH* is implied by the traditional one, and indeed equivalent to it. Lemma, The power set of any well orderable set is well orderable if and only if AC. This is one of the many equivalents of AC in ZF,due lo Rubin, 1960. Proposition. In ZF is a theorem: GCH* implies AC. Supose GCH*. Let A be a well orderable set; then |A| = K an ordinal cardinal, so A is equipotent with K and then P~A) is equipotent with P(K); therefore |P(A)I|= |P(K)| = 2K = K+. But then |P(A)|= K+ and P(A) 'is equipotent with K+ and K+ is an ordinal cardinal; therefore P(A) is well orderable with the well order induced by means of the bijection, from the well order of K+. Corolary: In ZF are theorems: GCH impIies AC and GCH is equivalent to GCH*. We see from this proof, that GCH asserts that the cardinal number of the power set of a well orderable set A is an ordinal, which is equivalent to AC, but GCH asserts also that that ordinal cardinal is |A|+ , the ordinal cardinal succesor of the ordinal cardinal of the well orderable set A.

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