NEW AXIOMS in SET THEORY Introduction on January 6Th, 1918
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Structural Reflection and the Ultimate L As the True, Noumenal Universe Of
Scuola Normale Superiore di Pisa CLASSE DI LETTERE Corso di Perfezionamento in Filosofia PHD degree in Philosophy Structural Reflection and the Ultimate L as the true, noumenal universe of mathematics Candidato: Relatore: Emanuele Gambetta Prof.- Massimo Mugnai anno accademico 2015-2016 Structural Reflection and the Ultimate L as the true noumenal universe of mathematics Emanuele Gambetta Contents 1. Introduction 7 Chapter 1. The Dream of Completeness 19 0.1. Preliminaries to this chapter 19 1. G¨odel'stheorems 20 1.1. Prerequisites to this section 20 1.2. Preliminaries to this section 21 1.3. Brief introduction to unprovable truths that are mathematically interesting 22 1.4. Unprovable mathematical statements that are mathematically interesting 27 1.5. Notions of computability, Turing's universe and Intuitionism 32 1.6. G¨odel'ssentences undecidable within PA 45 2. Transfinite Progressions 54 2.1. Preliminaries to this section 54 2.2. Gottlob Frege's definite descriptions and completeness 55 2.3. Transfinite progressions 59 3. Set theory 65 3.1. Preliminaries to this section 65 3.2. Prerequisites: ZFC axioms, ordinal and cardinal numbers 67 3.3. Reduction of all systems of numbers to the notion of set 71 3.4. The first large cardinal numbers and the Constructible universe L 76 3.5. Descriptive set theory, the axioms of determinacy and Luzin's problem formulated in second-order arithmetic 84 3 4 CONTENTS 3.6. The method of forcing and Paul Cohen's independence proof 95 3.7. Forcing Axioms, BPFA assumed as a phenomenal solution to the continuum hypothesis and a Kantian metaphysical distinction 103 3.8. -
Set-Theoretic Geology, the Ultimate Inner Model, and New Axioms
Set-theoretic Geology, the Ultimate Inner Model, and New Axioms Justin William Henry Cavitt (860) 949-5686 [email protected] Advisor: W. Hugh Woodin Harvard University March 20, 2017 Submitted in partial fulfillment of the requirements for the degree of Bachelor of Arts in Mathematics and Philosophy Contents 1 Introduction 2 1.1 Author’s Note . .4 1.2 Acknowledgements . .4 2 The Independence Problem 5 2.1 Gödelian Independence and Consistency Strength . .5 2.2 Forcing and Natural Independence . .7 2.2.1 Basics of Forcing . .8 2.2.2 Forcing Facts . 11 2.2.3 The Space of All Forcing Extensions: The Generic Multiverse 15 2.3 Recap . 16 3 Approaches to New Axioms 17 3.1 Large Cardinals . 17 3.2 Inner Model Theory . 25 3.2.1 Basic Facts . 26 3.2.2 The Constructible Universe . 30 3.2.3 Other Inner Models . 35 3.2.4 Relative Constructibility . 38 3.3 Recap . 39 4 Ultimate L 40 4.1 The Axiom V = Ultimate L ..................... 41 4.2 Central Features of Ultimate L .................... 42 4.3 Further Philosophical Considerations . 47 4.4 Recap . 51 1 5 Set-theoretic Geology 52 5.1 Preliminaries . 52 5.2 The Downward Directed Grounds Hypothesis . 54 5.2.1 Bukovský’s Theorem . 54 5.2.2 The Main Argument . 61 5.3 Main Results . 65 5.4 Recap . 74 6 Conclusion 74 7 Appendix 75 7.1 Notation . 75 7.2 The ZFC Axioms . 76 7.3 The Ordinals . 77 7.4 The Universe of Sets . 77 7.5 Transitive Models and Absoluteness . -
Set-Theoretic Foundations1
To appear in A. Caicedo et al, eds., Foundations of Mathematics, (Providence, RI: AMS). Set-theoretic Foundations1 It’s more or less standard orthodoxy these days that set theory - - ZFC, extended by large cardinals -- provides a foundation for classical mathematics. Oddly enough, it’s less clear what ‘providing a foundation’ comes to. Still, there are those who argue strenuously that category theory would do this job better than set theory does, or even that set theory can’t do it at all, and that category theory can. There are also those insist that set theory should be understood, not as the study of a single universe, V, purportedly described by ZFC + LCs, but as the study of a so-called ‘multiverse’ of set-theoretic universes -- while retaining its foundational role. I won’t pretend to sort out all these complex and contentious matters, but I do hope to compile a few relevant observations that might help bring illumination somewhat closer to hand. 1 It’s an honor to be included in this 60th birthday tribute to Hugh Woodin, who’s done so much to further, and often enough to re-orient, research on the fundamentals of contemporary set theory. I’m grateful to the organizers for this opportunity, and especially, to Professor Woodin for his many contributions. 2 I. Foundational uses of set theory The most common characterization of set theory’s foundational role, the characterization found in textbooks, is illustrated in the opening sentences of Kunen’s classic book on forcing: Set theory is the foundation of mathematics. All mathematical concepts are defined in terms of the primitive notions of set and membership. -
Axiomatic Set Teory P.D.Welch
Axiomatic Set Teory P.D.Welch. August 16, 2020 Contents Page 1 Axioms and Formal Systems 1 1.1 Introduction 1 1.2 Preliminaries: axioms and formal systems. 3 1.2.1 The formal language of ZF set theory; terms 4 1.2.2 The Zermelo-Fraenkel Axioms 7 1.3 Transfinite Recursion 9 1.4 Relativisation of terms and formulae 11 2 Initial segments of the Universe 17 2.1 Singular ordinals: cofinality 17 2.1.1 Cofinality 17 2.1.2 Normal Functions and closed and unbounded classes 19 2.1.3 Stationary Sets 22 2.2 Some further cardinal arithmetic 24 2.3 Transitive Models 25 2.4 The H sets 27 2.4.1 H - the hereditarily finite sets 28 2.4.2 H - the hereditarily countable sets 29 2.5 The Montague-Levy Reflection theorem 30 2.5.1 Absoluteness 30 2.5.2 Reflection Theorems 32 2.6 Inaccessible Cardinals 34 2.6.1 Inaccessible cardinals 35 2.6.2 A menagerie of other large cardinals 36 3 Formalising semantics within ZF 39 3.1 Definite terms and formulae 39 3.1.1 The non-finite axiomatisability of ZF 44 3.2 Formalising syntax 45 3.3 Formalising the satisfaction relation 46 3.4 Formalising definability: the function Def. 47 3.5 More on correctness and consistency 48 ii iii 3.5.1 Incompleteness and Consistency Arguments 50 4 The Constructible Hierarchy 53 4.1 The L -hierarchy 53 4.2 The Axiom of Choice in L 56 4.3 The Axiom of Constructibility 57 4.4 The Generalised Continuum Hypothesis in L. -
Are Large Cardinal Axioms Restrictive?
Are Large Cardinal Axioms Restrictive? Neil Barton∗ 24 June 2020y Abstract The independence phenomenon in set theory, while perva- sive, can be partially addressed through the use of large cardinal axioms. A commonly assumed idea is that large cardinal axioms are species of maximality principles. In this paper, I argue that whether or not large cardinal axioms count as maximality prin- ciples depends on prior commitments concerning the richness of the subset forming operation. In particular I argue that there is a conception of maximality through absoluteness, on which large cardinal axioms are restrictive. I argue, however, that large cardi- nals are still important axioms of set theory and can play many of their usual foundational roles. Introduction Large cardinal axioms are widely viewed as some of the best candi- dates for new axioms of set theory. They are (apparently) linearly ordered by consistency strength, have substantial mathematical con- sequences for questions independent from ZFC (such as consistency statements and Projective Determinacy1), and appear natural to the ∗Fachbereich Philosophie, University of Konstanz. E-mail: neil.barton@uni- konstanz.de. yI would like to thank David Aspero,´ David Fernandez-Bret´ on,´ Monroe Eskew, Sy-David Friedman, Victoria Gitman, Luca Incurvati, Michael Potter, Chris Scam- bler, Giorgio Venturi, Matteo Viale, Kameryn Williams and audiences in Cambridge, New York, Konstanz, and Sao˜ Paulo for helpful discussion. Two anonymous ref- erees also provided helpful comments, and I am grateful for their input. I am also very grateful for the generous support of the FWF (Austrian Science Fund) through Project P 28420 (The Hyperuniverse Programme) and the VolkswagenStiftung through the project Forcing: Conceptual Change in the Foundations of Mathematics. -
Lecture Notes: Axiomatic Set Theory
Lecture Notes: Axiomatic Set Theory Asaf Karagila Last Update: May 14, 2018 Contents 1 Introduction 3 1.1 Why do we need axioms?...............................3 1.2 Classes and sets.....................................4 1.3 The axioms of set theory................................5 2 Ordinals, recursion and induction7 2.1 Ordinals.........................................8 2.2 Transfinite induction and recursion..........................9 2.3 Transitive classes.................................... 10 3 The relative consistency of the Axiom of Foundation 12 4 Cardinals and their arithmetic 15 4.1 The definition of cardinals............................... 15 4.2 The Aleph numbers.................................. 17 4.3 Finiteness........................................ 18 5 Absoluteness and reflection 21 5.1 Absoluteness...................................... 21 5.2 Reflection........................................ 23 6 The Axiom of Choice 25 6.1 The Axiom of Choice.................................. 25 6.2 Weak version of the Axiom of Choice......................... 27 7 Sets of Ordinals 31 7.1 Cofinality........................................ 31 7.2 Some cardinal arithmetic............................... 32 7.3 Clubs and stationary sets............................... 33 7.4 The Club filter..................................... 35 8 Inner models of ZF 37 8.1 Inner models...................................... 37 8.2 Gödel’s constructible universe............................. 39 1 8.3 The properties of L ................................... 41 8.4 Ordinal definable sets................................. 42 9 Some combinatorics on ω1 43 9.1 Aronszajn trees..................................... 43 9.2 Diamond and Suslin trees............................... 44 10 Coda: Games and determinacy 46 2 Chapter 1 Introduction 1.1 Why do we need axioms? In modern mathematics, axioms are given to define an object. The axioms of a group define the notion of a group, the axioms of a Banach space define what it means for something to be a Banach space. -
The Logic of Sheaves, Sheaf Forcing and the Independence of The
The logic of sheaves, sheaf forcing and the independence of the Continuum Hypothesis J. Benavides∗ Abstract An introduction is given to the logic of sheaves of structures and to set theoretic forcing constructions based on this logic. Using these tools, it is presented an alternative proof of the independence of the Continuum Hypothesis; which simplifies and unifies the classical boolean and intuitionistic approaches, avoiding the difficulties linked to the categorical machinery of the topoi based approach. Introduction The first of the 10 problems that Hilbert posed in his famous conference the 1900 in Paris 1, the problem of the Continuum Hypothesis (CH for short), which asserts that every subset of the real numbers is either enumerable or has the cardinality of the Continuum; was the origin of probably one of the greatest achievements in Mathematics of the second half of the XX century: The forcing technique created by Paul Cohen during the sixties. Using this technique Cohen showed that the CH is independent (i.e. it cannot be proved or disproved) from the axioms of Zermelo-Fraenkel plus the Axiom of Choice (ZF C). At the beginning Cohen approached the problem in a pure syntactic way, he defined a procedure to find a contradiction in ZF C given a contradiction in ZF C + ¬CH; which implies that Con(ZF C) → Con(ZF C + ¬CH) (The consistency of ZF C implies the con- sistency of ZF C + ¬CH). However this approach was a non-constructivist one, for it was not given a model of ZF C + ¬CH. Nevertheless in his publish papers Cohen developed two arXiv:1111.5854v3 [math.LO] 7 Feb 2012 other approaches more constructivist. -
Regularity Properties and Determinacy
Regularity Properties and Determinacy MSc Thesis (Afstudeerscriptie) written by Yurii Khomskii (born September 5, 1980 in Moscow, Russia) under the supervision of Dr. Benedikt L¨owe, and submitted to the Board of Examiners in partial fulfillment of the requirements for the degree of MSc in Logic at the Universiteit van Amsterdam. Date of the public defense: Members of the Thesis Committee: August 14, 2007 Dr. Benedikt L¨owe Prof. Dr. Jouko V¨a¨an¨anen Prof. Dr. Joel David Hamkins Prof. Dr. Peter van Emde Boas Brian Semmes i Contents 0. Introduction............................ 1 1. Preliminaries ........................... 4 1.1 Notation. ........................... 4 1.2 The Real Numbers. ...................... 5 1.3 Trees. ............................. 6 1.4 The Forcing Notions. ..................... 7 2. ClasswiseConsequencesofDeterminacy . 11 2.1 Regularity Properties. .................... 11 2.2 Infinite Games. ........................ 14 2.3 Classwise Implications. .................... 16 3. The Marczewski-Burstin Algebra and the Baire Property . 20 3.1 MB and BP. ......................... 20 3.2 Fusion Sequences. ...................... 23 3.3 Counter-examples. ...................... 26 4. DeterminacyandtheBaireProperty.. 29 4.1 Generalized MB-algebras. .................. 29 4.2 Determinacy and BP(P). ................... 31 4.3 Determinacy and wBP(P). .................. 34 5. Determinacy andAsymmetric Properties. 39 5.1 The Asymmetric Properties. ................. 39 5.2 The General Definition of Asym(P). ............. 43 5.3 Determinacy and Asym(P). ................. 46 ii iii 0. Introduction One of the most intriguing developments of modern set theory is the investi- gation of two-player infinite games of perfect information. Of course, it is clear that applied game theory, as any other branch of mathematics, can be modeled in set theory. But we are talking about the converse: the use of infinite games as a tool to study fundamental set theoretic questions. -
FORCING and the INDEPENDENCE of the CONTINUUM HYPOTHESIS Contents 1. Preliminaries 2 1.1. Set Theory 2 1.2. Model Theory 3 2. Th
FORCING AND THE INDEPENDENCE OF THE CONTINUUM HYPOTHESIS F. CURTIS MASON Abstract. The purpose of this article is to develop the method of forcing and explain how it can be used to produce independence results. We first remind the reader of some basic set theory and model theory, which will then allow us to develop the logical groundwork needed in order to ensure that forcing constructions can in fact provide proper independence results. Next, we develop the basics of forcing, in particular detailing the construction of generic extensions of models of ZFC and proving that those extensions are themselves models of ZFC. Finally, we use the forcing notions Cκ and Kα to prove that the Continuum Hypothesis is independent from ZFC. Contents 1. Preliminaries 2 1.1. Set Theory 2 1.2. Model Theory 3 2. The Logical Justification for Forcing 4 2.1. The Reflection Principle 4 2.2. The Mostowski Collapsing Theorem and Countable Transitive Models 7 2.3. Basic Absoluteness Results 8 3. The Logical Structure of the Argument 9 4. Forcing Notions 9 5. Generic Extensions 12 6. The Forcing Relation 14 7. The Independence of the Continuum Hypothesis 18 Acknowledgements 23 References 23 The Continuum Hypothesis (CH) is the assertion that there are no cardinalities strictly in between that of the natural numbers and that of the reals, or more for- @0 mally, 2 = @1. In 1940, Kurt G¨odelshowed that both the Axiom of Choice and the Continuum Hypothesis are relatively consistent with the axioms of ZF ; he did this by constructing a so-called inner model L of the universe of sets V such that (L; 2) is a (class-sized) model of ZFC + CH. -
Determinacy and Large Cardinals
Determinacy and Large Cardinals Itay Neeman∗ Abstract. The principle of determinacy has been crucial to the study of definable sets of real numbers. This paper surveys some of the uses of determinacy, concentrating specifically on the connection between determinacy and large cardinals, and takes this connection further, to the level of games of length ω1. Mathematics Subject Classification (2000). 03E55; 03E60; 03E45; 03E15. Keywords. Determinacy, iteration trees, large cardinals, long games, Woodin cardinals. 1. Determinacy Let ωω denote the set of infinite sequences of natural numbers. For A ⊂ ωω let Gω(A) denote the length ω game with payoff A. The format of Gω(A) is displayed in Diagram 1. Two players, denoted I and II, alternate playing natural numbers forming together a sequence x = hx(n) | n < ωi in ωω called a run of the game. The run is won by player I if x ∈ A, and otherwise the run is won by player II. I x(0) x(2) ...... II x(1) x(3) ...... Diagram 1. The game Gω(A). A game is determined if one of the players has a winning strategy. The set A is ω determined if Gω(A) is determined. For Γ ⊂ P(ω ), det(Γ) denotes the statement that all sets in Γ are determined. Using the axiom of choice, or more specifically using a wellordering of the reals, it is easy to construct a non-determined set A. det(P(ωω)) is therefore false. On the other hand it has become clear through research over the years that det(Γ) is true if all the sets in Γ are definable by some concrete means. -
Sainsbury, M., Thinking About Things. Oxford: Oxford University Press, 2018, Pp
Sainsbury, M., Thinking About Things. Oxford: Oxford University Press, 2018, pp. ix + 199. Mark Sainsbury’s last book is about “aboutness”. It is widely recognised that in- tentionality—conceived as a peculiar property of mental states, i.e. their being about something—posits some of the most difficult philosophical issues. Moreo- ver, this mental phenomenon seems to be closely related to the linguistic phe- nomenon of intensionality, which has been extensively discussed in many areas of philosophy. Both intentionality and intensionality have something to do with nonexistent things, in so far as we can think and talk about things that do not exist—as fictional characters, mythical beasts, and contents of dreams and hal- lucinations. But how is it possible? And, more generally, how do intentional mental states actually work? In his book, Sainsbury addresses these fundamental problems within the framework of a representational theory of mind. Representations are nothing but what we think with (and, at least in typical cases, not what we think about). They can be characterised as concepts which behave “like words in the language of thought” (1). According to this view, in- tentional states are essentially related to representations, namely to concepts, and it seems quite intuitive to assume that not all concepts have objects: as a consequence, contra Brentano’s thesis,1 we should concede that some intentional states lack intentional objects. Now, Sainsbury raises four main questions about intentionality: 1. How are intentional mental states attributed? 2. What does their “aboutness" consist in? 3. Are they (always) relational? 4. Does any of them require there to be nonexistent things? Given what we just said, Sainsbury’s answer to (4) will reasonably be negative. -
Ralf Schindler Exploring Independence and Truth
Universitext Ralf Schindler Set Theory Exploring Independence and Truth Universitext Universitext Series editors Sheldon Axler San Francisco State University, San Francisco, CA, USA Vincenzo Capasso Università degli Studi di Milano, Milan, Italy Carles Casacuberta Universitat de Barcelona, Barcelona, Spain Angus MacIntyre Queen Mary University of London, London, UK Kenneth Ribet University of California, Berkeley, CA, USA Claude Sabbah CNRS École Polytechnique Centre de mathématiques, Palaiseau, France Endre Süli University of Oxford, Oxford, UK Wojbor A. Woyczynski Case Western Reserve University, Cleveland, OH, USA Universitext is a series of textbooks that presents material from a wide variety of mathematical disciplines at master’s level and beyond. The books, often well class-tested by their author, may have an informal, personal, even experimental approach to their subject matter. Some of the most successful and established books in the series have evolved through several editions, always following the evolution of teaching curricula, into very polished texts. Thus as research topics trickle down into graduate-level teaching, first textbooks written for new, cutting-edge courses may make their way into Universitext. For further volumes: http://www.springer.com/series/223 Ralf Schindler Set Theory Exploring Independence and Truth 123 Ralf Schindler Institut für Mathematische Logik und Grundlagenforschung Universität Münster Münster Germany ISSN 0172-5939 ISSN 2191-6675 (electronic) ISBN 978-3-319-06724-7 ISBN 978-3-319-06725-4 (eBook) DOI 10.1007/978-3-319-06725-4 Springer Cham Heidelberg New York Dordrecht London Library of Congress Control Number: 2014938475 Mathematics Subject Classification: 03-01, 03E10, 03E15, 03E35, 03E45, 03E55, 03E60 Ó Springer International Publishing Switzerland 2014 This work is subject to copyright.