Coordinatisation and canonical bases in simple theories

2000 ◽  
Vol 65 (1) ◽  
pp. 293-309 ◽  
Author(s):  
Bradd Hart ◽  
Byunghan Kim ◽  
Anand Pillay
Keyword(s):  
General Theory ◽  
Model Theory ◽  
Equivalence Relation ◽  
Finite Rank ◽  
Stability Theory ◽  
Saturated Model ◽  
Order Model ◽  
Canonical Bases ◽  
Simple Theories ◽  
First Order

In this paper we discuss several generalization of theorems from stability theory to simple theories. Cherlin and Hrushovski, in [2] develop a substitute for canonical bases in finite rank, ω-categorical supersimple theories. Motivated by methods there, we prove the existence of canonical bases (in a suitable sense) for types in any simple theory. This is done in Section 2. In general these canonical bases will (as far as we know) exist only as “hyperimaginaries”, namely objects of the form a/E where a is a possibly infinite tuple and E a type-definable equivalence relation. (In the supersimple, ω-categorical case, these reduce to ordinary imaginaries.) So in Section 1 we develop the general theory of hyperimaginaries and show how first order model theory (including the theory of forking) generalises to hyperimaginaries. We go on, in Section 3 to show the existence and ubiquity of regular types in supersimple theories, ω-categorical simple structures and modularity is discussed in Section 4. It is also shown here how the general machinery of simplicity simplifies some of the general theory of smoothly approximable (or Lie-coordinatizable) structures from [2].Throughout this paper we will work in a large, saturated model M of a complete theory T. All types, sets and sequences will have size smaller than the size of M. We will assume that the reader is familiar with the basics of forking in simple theories as laid out in [4] and [6]. For basic stability-theoretic results concerning regular types, orthogonality etc., see [1] or [9].

scholarly journals Compactness and Independence in Non First Order Frameworks

2005 ◽  
Vol 11 (1) ◽  
pp. 28-50 ◽  
Author(s):  
Itay Ben-Yaacov
Keyword(s):  
Model Theory ◽  
Finite Cover ◽  
Order Model ◽  
Simple Theories ◽  
First Order ◽  
The Cost

AbstractThis communication deals with positive model theory, a non first order model theoretic setting which preserves compactness at the cost of giving up negation. Positive model theory deals transparently with hyperimaginaries, and accommodates various analytic structures which defy direct first order treatment. We describe the development of simplicity theory in this setting, and an application to the lovely pairs of models of simple theories without the weak non finite cover property.

An introduction to recursively saturated and resplendent models

1976 ◽  
Vol 41 (2) ◽  
pp. 531-536 ◽  
Author(s):  
Jon Barwise ◽  
John Schlipf
Keyword(s):  
Set Theory ◽  
Model Theory ◽  
Order Logic ◽  
First Order Logic ◽  
Saturated Model ◽  
Order Model ◽  
Special Model ◽  
First Order ◽  
Admissible Sets ◽  
Recursively Saturated

The notions of recursively saturated and resplendent models grew out of the study of admissible sets with urelements and admissible fragments of Lω1ω, but, when applied to ordinary first order model theory, give us new tools for research and exposition. We will discuss their history in §3.The notion of saturated model has proven to be important in model theory. Its most important property for applications is that if , are saturated and of the same cardinality then = iff ≅ . See, e.g., Chang-Keisler [3]. The main drawback is that saturated models exist only under unusual assumptions of set theory. For example, if 2κ = κ+ then every theory T of L has a saturated model of power κ+. (Similarly, if κ is strongly inaccessible, then every T has a saturated model of power κ.) On the other hand, a theory T like Peano arithmetic, with types, cannot have a saturated model in any power κ with ω ≤ κ ≤ .One method for circumventing these problems of existence (or rather non-existence) is the use of “special” models (cf. [3]). If κ = Σλ<κ2λ, κ < ω, then every theory T of L has a special model of power κ. Such cardinals are large and, themselves, rather special. There are definite aesthetic objections to the use of these large, singular models to prove results about first order logic.

scholarly journals First Order Languages: Further Syntax and Semantics

2011 ◽  
Vol 19 (3) ◽  
pp. 179-192 ◽  
Author(s):  
Marco Caminati
Keyword(s):  
Model Theory ◽  
Atomic Formula ◽  
Order Model ◽  
Logical Implication ◽  
First Order ◽  
Definition Of ◽  
Theory Interpretation

First Order Languages: Further Syntax and SemanticsThird of a series of articles laying down the bases for classical first order model theory. Interpretation of a language in a universe set. Evaluation of a term in a universe. Truth evaluation of an atomic formula. Reassigning the value of a symbol in a given interpretation. Syntax and semantics of a non atomic formula are then defined concurrently (this point is explained in [16], 4.2.1). As a consequence, the evaluation of any w.f.f. string and the relation of logical implication are introduced. Depth of a formula. Definition of satisfaction and entailment (aka entailment or logical implication) relations, see [18] III.3.2 and III.4.1 respectively.

THE FIRST-ORDER THEORY OF BRANCH GROUPS

2016 ◽  
Vol 102 (1) ◽  
pp. 150-158 ◽  
Author(s):  
JOHN S. WILSON
Keyword(s):  
Model Theory ◽  
Order Theory ◽  
Order Model ◽  
First Order ◽  
First Order Theory

It is shown that for many branch groups $G$ the action on the ambient tree can be interpreted in $G$, in the sense of first-order model theory.

Infinitary analogs of theorems from first order model theory

1971 ◽  
Vol 36 (2) ◽  
pp. 216-228 ◽  
Author(s):  
Jerome Malitz
Keyword(s):  
Model Theory ◽  
Expressive Power ◽  
Direct Products ◽  
Order Model ◽  
Relation Symbol ◽  
First Order ◽  
Identity Symbol

The material presented here belongs to the model theory of the Lκ, λ languages. Our results are either infinitary analogs of important theorems in finitary model theory, or else show that such analogs do not exist.For example, it is well known that whenever i, and , have the same true Lω, ω sentences (i.e., are elementarily equivalent) for i = 1, 2, then the cardinal sums 1 + 2 and + have the same true Lω, ω sentences, and the direct products 1 · 2 and · have the same true Lω, ω sentences [3]. We show that this is true when ‘Lω, ω’ is replaced by ‘Lκ, λ’ if and only if κ is strongly inaccessible. For Lω1, ω this settles a question, posed by Lopez-Escobar [7].In §3 we give a complete description of the expressive power of those sentences of Lκ, λ in which the identity symbol is the only relation symbol which occurs. This extends a result by Hanf [4].

On nonstandard models in higher order logic

1984 ◽  
Vol 49 (1) ◽  
pp. 204-219
Author(s):  
Christian Hort ◽  
Horst Osswald
Keyword(s):  
Model Theory ◽  
Type Theory ◽  
Order Logic ◽  
Elementary Embedding ◽  
Simple Type ◽  
Order Model ◽  
Higher Order Logic ◽  
First Order ◽  
Simple Type Theory ◽  
Nonstandard Models

There are two concepts of standard/nonstandard models in simple type theory.The first concept—we might call it the pragmatical one—interprets type theory as a first order logic with countably many sorts of variables: the variables for the urelements of type 0,…, the n-ary relational variables of type (τ1, …, τn) with arguments of type (τ1,…,τn), respectively. If A ≠ ∅ then 〈Aτ〉 is called a model of type logic, if A0 = A and . 〈Aτ〉 is called full if, for every τ = (τ1,…,τn), . The variables for the urelements range over the elements of A and the variables of type (τ1,…, τn) range over those subsets of which are elements of . The theory Th(〈Aτ〉) is the set of all closed formulas in the language which hold in 〈Aτ〉 under natural interpretation of the constants. If 〈Bτ〉 is a model of Th(〈Aτ〉), then there exists a sequence 〈fτ〉 of functions fτ: Aτ → Bτ such that 〈fτ〉 is an elementary embedding from 〈Aτ〉 into 〈Bτ〉. 〈Bτ〉 is called a nonstandard model of 〈Aτ〉, if f0 is not surjective. Otherwise 〈Bτ〉 is called a standard model of 〈Aτ〉.This first concept of model theory in type logic seems to be preferable for applications in model theory, for example in nonstandard analysis, since all nice properties of first order model theory (completeness, compactness, and so on) are preserved.

scholarly journals SPACES OF TYPES IN POSITIVE MODEL THEORY

2019 ◽  
Vol 84 (02) ◽  
pp. 833-848
Author(s):  
LEVON HAYKAZYAN
Keyword(s):  
Model Theory ◽  
Stone Space ◽  
Distributive Lattices ◽  
Stone Duality ◽  
Order Model ◽  
First Order ◽  
Countable Models

AbstractWe introduce a notion of the space of types in positive model theory based on Stone duality for distributive lattices. We show that this space closely mirrors the Stone space of types in the full first-order model theory with negation (Tarskian model theory). We use this to generalise some classical results on countable models from the Tarskian setting to positive model theory.

From Stability to Simplicity

1998 ◽  
Vol 4 (1) ◽  
pp. 17-36 ◽  
Author(s):  
Byunghan Kim ◽  
Anand Pillay
Keyword(s):  
Recent Work ◽  
Stability Theory ◽  
Simple Theories ◽  
First Order ◽  
Closed Fields ◽  
Exhaustive Study ◽  
Pseudofinite Fields ◽  
Algebraically Closed Fields ◽  
Major Progress ◽  
Made In

§1. Introduction. In this report we wish to describe recent work on a class of first order theories first introduced by Shelah in [32], the simple theories. Major progress was made in the first author's doctoral thesis [17]. We will give a survey of this, as well as further works by the authors and others.The class of simple theories includes stable theories, but also many more, such as the theory of the random graph. Moreover, many of the theories of particular algebraic structures which have been studied recently (pseudofinite fields, algebraically closed fields with a generic automorphism, smoothly approximable structures) turn out to be simple. The interest is basically that a large amount of the machinery of stability theory, invented by Shelah, is valid in the broader class of simple theories. Stable theories will be defined formally in the next section. An exhaustive study of them is carried out in [33]. Without trying to read Shelah's mind, we feel comfortable in saying that the importance of stability for Shelah lay partly in the fact that an unstable theory T has 2λ many models in any cardinal λ ≥ ω1 + |T| (proved by Shelah). (Note that for λ ≥ |T| 2λ is the maximum possible number of models of cardinality λ.)

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