An Introduction to the Axiom of Choice
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Mathematics 144 Set Theory Fall 2012 Version
MATHEMATICS 144 SET THEORY FALL 2012 VERSION Table of Contents I. General considerations.……………………………………………………………………………………………………….1 1. Overview of the course…………………………………………………………………………………………………1 2. Historical background and motivation………………………………………………………….………………4 3. Selected problems………………………………………………………………………………………………………13 I I. Basic concepts. ………………………………………………………………………………………………………………….15 1. Topics from logic…………………………………………………………………………………………………………16 2. Notation and first steps………………………………………………………………………………………………26 3. Simple examples…………………………………………………………………………………………………………30 I I I. Constructions in set theory.………………………………………………………………………………..……….34 1. Boolean algebra operations.……………………………………………………………………………………….34 2. Ordered pairs and Cartesian products……………………………………………………………………… ….40 3. Larger constructions………………………………………………………………………………………………..….42 4. A convenient assumption………………………………………………………………………………………… ….45 I V. Relations and functions ……………………………………………………………………………………………….49 1.Binary relations………………………………………………………………………………………………………… ….49 2. Partial and linear orderings……………………………..………………………………………………… ………… 56 3. Functions…………………………………………………………………………………………………………… ….…….. 61 4. Composite and inverse function.…………………………………………………………………………… …….. 70 5. Constructions involving functions ………………………………………………………………………… ……… 77 6. Order types……………………………………………………………………………………………………… …………… 80 i V. Number systems and set theory …………………………………………………………………………………. 84 1. The Natural Numbers and Integers…………………………………………………………………………….83 2. Finite induction -
Biography Paper – Georg Cantor
Mike Garkie Math 4010 – History of Math UCD Denver 4/1/08 Biography Paper – Georg Cantor Few mathematicians are house-hold names; perhaps only Newton and Euclid would qualify. But there is a second tier of mathematicians, those whose names might not be familiar, but whose discoveries are part of everyday math. Examples here are Napier with logarithms, Cauchy with limits and Georg Cantor (1845 – 1918) with sets. In fact, those who superficially familier with Georg Cantor probably have two impressions of the man: First, as a consequence of thinking about sets, Cantor developed a theory of the actual infinite. And second, that Cantor was a troubled genius, crippled by Freudian conflict and mental illness. The first impression is fundamentally true. Cantor almost single-handedly overturned the Aristotle’s concept of the potential infinite by developing the concept of transfinite numbers. And, even though Bolzano and Frege made significant contributions, “Set theory … is the creation of one person, Georg Cantor.” [4] The second impression is mostly false. Cantor certainly did suffer from mental illness later in his life, but the other emotional baggage assigned to him is mostly due his early biographers, particularly the infamous E.T. Bell in Men Of Mathematics [7]. In the racially charged atmosphere of 1930’s Europe, the sensational story mathematician who turned the idea of infinity on its head and went crazy in the process, probably make for good reading. The drama of the controversy over Cantor’s ideas only added spice. 1 Fortunately, modern scholars have corrected the errors and biases in older biographies. -
Algebra I Chapter 1. Basic Facts from Set Theory 1.1 Glossary of Abbreviations
Notes: c F.P. Greenleaf, 2000-2014 v43-s14sets.tex (version 1/1/2014) Algebra I Chapter 1. Basic Facts from Set Theory 1.1 Glossary of abbreviations. Below we list some standard math symbols that will be used as shorthand abbreviations throughout this course. means “for all; for every” • ∀ means “there exists (at least one)” • ∃ ! means “there exists exactly one” • ∃ s.t. means “such that” • = means “implies” • ⇒ means “if and only if” • ⇐⇒ x A means “the point x belongs to a set A;” x / A means “x is not in A” • ∈ ∈ N denotes the set of natural numbers (counting numbers) 1, 2, 3, • · · · Z denotes the set of all integers (positive, negative or zero) • Q denotes the set of rational numbers • R denotes the set of real numbers • C denotes the set of complex numbers • x A : P (x) If A is a set, this denotes the subset of elements x in A such that •statement { ∈ P (x)} is true. As examples of the last notation for specifying subsets: x R : x2 +1 2 = ( , 1] [1, ) { ∈ ≥ } −∞ − ∪ ∞ x R : x2 +1=0 = { ∈ } ∅ z C : z2 +1=0 = +i, i where i = √ 1 { ∈ } { − } − 1.2 Basic facts from set theory. Next we review the basic definitions and notations of set theory, which will be used throughout our discussions of algebra. denotes the empty set, the set with nothing in it • ∅ x A means that the point x belongs to a set A, or that x is an element of A. • ∈ A B means A is a subset of B – i.e. -
MATH 361 Homework 9
MATH 361 Homework 9 Royden 3.3.9 First, we show that for any subset E of the real numbers, Ec + y = (E + y)c (translating the complement is equivalent to the complement of the translated set). Without loss of generality, assume E can be written as c an open interval (e1; e2), so that E + y is represented by the set fxjx 2 (−∞; e1 + y) [ (e2 + y; +1)g. This c is equal to the set fxjx2 = (e1 + y; e2 + y)g, which is equivalent to the set (E + y) . Second, Let B = A − y. From Homework 8, we know that outer measure is invariant under translations. Using this along with the fact that E is measurable: m∗(A) = m∗(B) = m∗(B \ E) + m∗(B \ Ec) = m∗((B \ E) + y) + m∗((B \ Ec) + y) = m∗(((A − y) \ E) + y) + m∗(((A − y) \ Ec) + y) = m∗(A \ (E + y)) + m∗(A \ (Ec + y)) = m∗(A \ (E + y)) + m∗(A \ (E + y)c) The last line follows from Ec + y = (E + y)c. Royden 3.3.10 First, since E1;E2 2 M and M is a σ-algebra, E1 [ E2;E1 \ E2 2 M. By the measurability of E1 and E2: ∗ ∗ ∗ c m (E1) = m (E1 \ E2) + m (E1 \ E2) ∗ ∗ ∗ c m (E2) = m (E2 \ E1) + m (E2 \ E1) ∗ ∗ ∗ ∗ c ∗ c m (E1) + m (E2) = 2m (E1 \ E2) + m (E1 \ E2) + m (E1 \ E2) ∗ ∗ ∗ c ∗ c = m (E1 \ E2) + [m (E1 \ E2) + m (E1 \ E2) + m (E1 \ E2)] c c Second, E1 \ E2, E1 \ E2, and E1 \ E2 are disjoint sets whose union is equal to E1 [ E2. -
SRB MEASURES for ALMOST AXIOM a DIFFEOMORPHISMS José Alves, Renaud Leplaideur
SRB MEASURES FOR ALMOST AXIOM A DIFFEOMORPHISMS José Alves, Renaud Leplaideur To cite this version: José Alves, Renaud Leplaideur. SRB MEASURES FOR ALMOST AXIOM A DIFFEOMORPHISMS. 2013. hal-00778407 HAL Id: hal-00778407 https://hal.archives-ouvertes.fr/hal-00778407 Preprint submitted on 20 Jan 2013 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. SRB MEASURES FOR ALMOST AXIOM A DIFFEOMORPHISMS JOSE´ F. ALVES AND RENAUD LEPLAIDEUR Abstract. We consider a diffeomorphism f of a compact manifold M which is Almost Axiom A, i.e. f is hyperbolic in a neighborhood of some compact f-invariant set, except in some singular set of neutral points. We prove that if there exists some f-invariant set of hyperbolic points with positive unstable-Lebesgue measure such that for every point in this set the stable and unstable leaves are \long enough", then f admits a probability SRB measure. Contents 1. Introduction 2 1.1. Background 2 1.2. Statement of results 2 1.3. Overview 4 2. Markov rectangles 5 2.1. Neighborhood of critical zone 5 2.2. First generation of rectangles 6 2.3. Second generation rectangles 7 2.4. -
Solutions to Homework 2 Math 55B
Solutions to Homework 2 Math 55b 1. Give an example of a subset A ⊂ R such that, by repeatedly taking closures and complements. Take A := (−5; −4)−Q [[−4; −3][f2g[[−1; 0][(1; 2)[(2; 3)[ (4; 5)\Q . (The number of intervals in this sets is unnecessarily large, but, as Kevin suggested, taking a set that is complement-symmetric about 0 cuts down the verification in half. Note that this set is the union of f0g[(1; 2)[(2; 3)[ ([4; 5] \ Q) and the complement of the reflection of this set about the the origin). Because of this symmetry, we only need to verify that the closure- complement-closure sequence of the set f0g [ (1; 2) [ (2; 3) [ (4; 5) \ Q consists of seven distinct sets. Here are the (first) seven members of this sequence; from the eighth member the sets start repeating period- ically: f0g [ (1; 2) [ (2; 3) [ (4; 5) \ Q ; f0g [ [1; 3] [ [4; 5]; (−∞; 0) [ (0; 1) [ (3; 4) [ (5; 1); (−∞; 1] [ [3; 4] [ [5; 1); (1; 3) [ (4; 5); [1; 3] [ [4; 5]; (−∞; 1) [ (3; 4) [ (5; 1). Thus, by symmetry, both the closure- complement-closure and complement-closure-complement sequences of A consist of seven distinct sets, and hence there is a total of 14 different sets obtained from A by taking closures and complements. Remark. That 14 is the maximal possible number of sets obtainable for any metric space is the Kuratowski complement-closure problem. This is proved by noting that, denoting respectively the closure and com- plement operators by a and b and by e the identity operator, the relations a2 = a; b2 = e, and aba = abababa take place, and then one can simply list all 14 elements of the monoid ha; b j a2 = a; b2 = e; aba = abababai. -
Bounding, Splitting and Almost Disjointness
Bounding, splitting and almost disjointness Jonathan Schilhan Advisor: Vera Fischer Kurt G¨odelResearch Center Fakult¨atf¨urMathematik, Universit¨atWien Abstract This bachelor thesis provides an introduction to set theory, cardinal character- istics of the continuum and the generalized cardinal characteristics on arbitrary regular cardinals. It covers the well known ZFC relations of the bounding, the dominating, the almost disjointness and the splitting number as well as some more recent results about their generalizations to regular uncountable cardinals. We also present an overview on the currently known consistency results and formulate open questions. 1 CONTENTS Contents Introduction 3 1 Prerequisites 5 1.1 Model Theory/Predicate Logic . .5 1.2 Ordinals/Cardinals . .6 1.2.1 Ordinals . .7 1.2.2 Cardinals . .9 1.2.3 Regular Cardinals . 10 2 Cardinal Characteristics of the continuum 11 2.1 The bounding and the dominating number . 11 2.2 Splitting and mad families . 13 3 Cardinal Characteristics on regular uncountable cardinals 16 3.1 Generalized bounding, splitting and almost disjointness . 16 3.2 An unexpected inequality between the bounding and splitting numbers . 18 3.2.1 Elementary Submodels . 18 3.2.2 Filters and Ideals . 19 3.2.3 Proof of Theorem 3.2.1 (s(κ) ≤ b(κ)) . 20 3.3 A relation between generalized dominating and mad families . 23 References 28 2 Introduction @0 Since it was proven that the continuum hypothesis (2 = @1) is independent of the axioms of ZFC, the natural question arises what value 2@0 could take and it was shown @0 that 2 could be consistently nearly anything. -
Almost-Free E-Rings of Cardinality ℵ1 Rudige¨ R Gob¨ El, Saharon Shelah and Lutz Strung¨ Mann
Sh:785 Canad. J. Math. Vol. 55 (4), 2003 pp. 750–765 Almost-Free E-Rings of Cardinality @1 Rudige¨ r Gob¨ el, Saharon Shelah and Lutz Strung¨ mann Abstract. An E-ring is a unital ring R such that every endomorphism of the underlying abelian group R+ is multiplication by some ring element. The existence of almost-free E-rings of cardinality greater @ than 2 0 is undecidable in ZFC. While they exist in Godel'¨ s universe, they do not exist in other models @ of set theory. For a regular cardinal @1 ≤ λ ≤ 2 0 we construct E-rings of cardinality λ in ZFC which have @1-free additive structure. For λ = @1 we therefore obtain the existence of almost-free E-rings of cardinality @1 in ZFC. 1 Introduction + Recall that a unital ring R is an E-ring if the evaluation map ": EndZ(R ) ! R given by ' 7! '(1) is a bijection. Thus every endomorphism of the abelian group R+ is multiplication by some element r 2 R. E-rings were introduced by Schultz [20] and easy examples are subrings of the rationals Q or pure subrings of the ring of p-adic integers. Schultz characterized E-rings of finite rank and the books by Feigelstock [9, 10] and an article [18] survey the results obtained in the eighties, see also [8, 19]. In a natural way the notion of E-rings extends to modules by calling a left R-module M an E(R)-module or just E-module if HomZ(R; M) = HomR(R; M) holds, see [1]. -
ON the CONSTRUCTION of NEW TOPOLOGICAL SPACES from EXISTING ONES Consider a Function F
ON THE CONSTRUCTION OF NEW TOPOLOGICAL SPACES FROM EXISTING ONES EMILY RIEHL Abstract. In this note, we introduce a guiding principle to define topologies for a wide variety of spaces built from existing topological spaces. The topolo- gies so-constructed will have a universal property taking one of two forms. If the topology is the coarsest so that a certain condition holds, we will give an elementary characterization of all continuous functions taking values in this new space. Alternatively, if the topology is the finest so that a certain condi- tion holds, we will characterize all continuous functions whose domain is the new space. Consider a function f : X ! Y between a pair of sets. If Y is a topological space, we could define a topology on X by asking that it is the coarsest topology so that f is continuous. (The finest topology making f continuous is the discrete topology.) Explicitly, a subbasis of open sets of X is given by the preimages of open sets of Y . With this definition, a function W ! X, where W is some other space, is continuous if and only if the composite function W ! Y is continuous. On the other hand, if X is assumed to be a topological space, we could define a topology on Y by asking that it is the finest topology so that f is continuous. (The coarsest topology making f continuous is the indiscrete topology.) Explicitly, a subset of Y is open if and only if its preimage in X is open. With this definition, a function Y ! Z, where Z is some other space, is continuous if and only if the composite function X ! Z is continuous. -
MAD FAMILIES CONSTRUCTED from PERFECT ALMOST DISJOINT FAMILIES §1. Introduction. a Family a of Infinite Subsets of Ω Is Called
The Journal of Symbolic Logic Volume 00, Number 0, XXX 0000 MAD FAMILIES CONSTRUCTED FROM PERFECT ALMOST DISJOINT FAMILIES JORG¨ BRENDLE AND YURII KHOMSKII 1 Abstract. We prove the consistency of b > @1 together with the existence of a Π1- definable mad family, answering a question posed by Friedman and Zdomskyy in [7, Ques- tion 16]. For the proof we construct a mad family in L which is an @1-union of perfect a.d. sets, such that this union remains mad in the iterated Hechler extension. The construction also leads us to isolate a new cardinal invariant, the Borel almost-disjointness number aB , defined as the least number of Borel a.d. sets whose union is a mad family. Our proof yields the consistency of aB < b (and hence, aB < a). x1. Introduction. A family A of infinite subsets of ! is called almost disjoint (a.d.) if any two elements a; b of A have finite intersection. A family A is called maximal almost disjoint, or mad, if it is an infinite a.d. family which is maximal with respect to that property|in other words, 8a 9b 2 A (ja \ bj = !). The starting point of this paper is the following theorem of Adrian Mathias [11, Corollary 4.7]: Theorem 1.1 (Mathias). There are no analytic mad families. 1 On the other hand, it is easy to see that in L there is a Σ2 definable mad family. In [12, Theorem 8.23], Arnold Miller used a sophisticated method to 1 prove the seemingly stronger result that in L there is a Π1 definable mad family. -
The Axiom of Choice and Its Implications
THE AXIOM OF CHOICE AND ITS IMPLICATIONS KEVIN BARNUM Abstract. In this paper we will look at the Axiom of Choice and some of the various implications it has. These implications include a number of equivalent statements, and also some less accepted ideas. The proofs discussed will give us an idea of why the Axiom of Choice is so powerful, but also so controversial. Contents 1. Introduction 1 2. The Axiom of Choice and Its Equivalents 1 2.1. The Axiom of Choice and its Well-known Equivalents 1 2.2. Some Other Less Well-known Equivalents of the Axiom of Choice 3 3. Applications of the Axiom of Choice 5 3.1. Equivalence Between The Axiom of Choice and the Claim that Every Vector Space has a Basis 5 3.2. Some More Applications of the Axiom of Choice 6 4. Controversial Results 10 Acknowledgments 11 References 11 1. Introduction The Axiom of Choice states that for any family of nonempty disjoint sets, there exists a set that consists of exactly one element from each element of the family. It seems strange at first that such an innocuous sounding idea can be so powerful and controversial, but it certainly is both. To understand why, we will start by looking at some statements that are equivalent to the axiom of choice. Many of these equivalences are very useful, and we devote much time to one, namely, that every vector space has a basis. We go on from there to see a few more applications of the Axiom of Choice and its equivalents, and finish by looking at some of the reasons why the Axiom of Choice is so controversial. -
Cardinality of Wellordered Disjoint Unions of Quotients of Smooth Equivalence Relations on R with All Classes Countable Is the Main Result of the Paper
CARDINALITY OF WELLORDERED DISJOINT UNIONS OF QUOTIENTS OF SMOOTH EQUIVALENCE RELATIONS WILLIAM CHAN AND STEPHEN JACKSON Abstract. Assume ZF + AD+ + V = L(P(R)). Let ≈ denote the relation of being in bijection. Let κ ∈ ON and hEα : α<κi be a sequence of equivalence relations on R with all classes countable and for all α<κ, R R R R R /Eα ≈ . Then the disjoint union Fα<κ /Eα is in bijection with × κ and Fα<κ /Eα has the J´onsson property. + <ω Assume ZF + AD + V = L(P(R)). A set X ⊆ [ω1] 1 has a sequence hEα : α<ω1i of equivalence relations on R such that R/Eα ≈ R and X ≈ F R/Eα if and only if R ⊔ ω1 injects into X. α<ω1 ω ω Assume AD. Suppose R ⊆ [ω1] × R is a relation such that for all f ∈ [ω1] , Rf = {x ∈ R : R(f,x)} ω is nonempty and countable. Then there is an uncountable X ⊆ ω1 and function Φ : [X] → R which uniformizes R on [X]ω: that is, for all f ∈ [X]ω, R(f, Φ(f)). Under AD, if κ is an ordinal and hEα : α<κi is a sequence of equivalence relations on R with all classes ω R countable, then [ω1] does not inject into Fα<κ /Eα. 1. Introduction The original motivation for this work comes from the study of a simple combinatorial property of sets using only definable methods. The combinatorial property of concern is the J´onsson property: Let X be any n n <ω n set.