Model theory is the study of (classes of) mathematical structures (e.g. groups, fields, graphs, universes of set theory) using tools from mathematical logic. Objects of study in model theory are models for formal languages which are structures that give meaning to the sentences of these formal ...

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Logical Consequences and Ordered Fields.

How do I show that these two: $1.$ $\forall x(0 < x \rightarrow (-x) < 0)$ $2.$ $\forall x \forall y \forall z((x<y \wedge z<0) \rightarrow (y *z) <(x*z))$ are logical consequences ...
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Exercise 20.7 of Sacks's Saturated Model Theory (Partial isomorphisms)

I'm trying to solve the exercise in the title and I think it makes no sense. Here's what it says: An onto map $f: X \to Y$ is called an elementary partial isomorphism between $\mathcal{A}$ and ...
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Non-forking frames in AEC

Here http://shelah.logic.at/files/875.pdf on page 15, item 4 in the proof of 2.2.6, I would like to know why $S(M)\leq \lambda \times \lambda^+$. I understand that models in $K$ have cardinality ...
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An affine group behaving like a field

This question is about an example of interpreting a field in an affine group, from Section 1.3 of Marker's Model theory: An introduction. Let $F$ be an infinite field and $G$ be the group of ...
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Natural Algebraic Structures on the Set of Automorphisms of a Structure

If $M$ is a first order structure (e.g. some algebraic structure) we usually refer to its set of automorphisms, $Aut(M)$, as a group with its natural "function combination" operator.i.e. $\langle ...
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Model theory of valued

I am currently reading these notes on model theory of valued fields, in the section 3.3 appears this theorem: Theorem. Let $K$ and $L$ be valued fields, with residue fields $k_K$ and $k_L$ ...
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38 views

Relations between equations in a theory, and the number of independent equations

I have a question on equational reasoning in theories, which is made quite often in mathmeatics, and I am trying to make this more formal. So for my attempt to make this more rigouros, I choosed ...
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Books and articles on model theory for set theory

I'm interested in books and/or articles which explore a little more in depth the model theory of set theory. I'm aware that most books on set theory have a section or two on models (e.g. Jech, Kunen), ...
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Type-definable Forcing or forcing in a non-first order setting

Roughly speaking, in set forcing the forcing notion is a set from ground model's perspective and in class forcing its a definable subset of the ground model given by solutions of some formula with ...
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63 views

Reference request basic logic/model theory

I'm taking a knowledge representation class and need more perspective on basic model theory. We're currently using Levesque and Brachman. Specifically, a question on the midterm was something like, ...
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Cardinality of the set of non isomorphic structures of fixed cardinality

Let $L$, be a language and $\alpha$ be a cardinal; let $\Gamma:= \{\text{set of non isomorphic $L$ structures, having cardinality $\alpha$}\}$. Prove that $\operatorname{Card}(\Gamma)\leq ...
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Colored operads as finitely essentially algebraic theory.

I call a planar operad what is also called planar (multi-)coloured operad or multicategory and symmetric operad a symmetric multicategory or symmetric (multi-)colored operad. I have two questions ...
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Characterization of a theory whose model has elementary submodels as only its submodels

This is a problem (2.5.12) from Marker's Model Theory: An Introduction of showing that a model has only elementary submodels as its submodels if and only if for every formula is equivalent to some ...
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24 views

$n$-types of a structure

I got introduced to $n$-types of a structure a few weeks ago, but I can't really get my head around it. In an exercise I am asked the following: Define the binary relation $=_2$ on $\mathbb Z$ by: ...
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38 views

Proving the completeness of a theory $\Gamma$

Given a set of sentences $\Gamma$ in a first-order-language $\mathcal{L}$, such that for all structures $\mathcal{A}=(A,\ldots)$ and $\mathcal{B}=(B,\ldots)$, if both $\mathcal{A}$ and ${\cal B}$ ...
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Marker Exercise 2.5.10: universal part of a theory and supermodel

I'm trying to solve Exercise 2.5.10 in Marker's Model Theory: An Introduction. It goes: Let T be an $\mathcal L$-theory and $T_\forall$ be all of the universal sentences $\phi$ such that $T ...
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Any substructure of $(\mathbb{N}; 0, 1, +, \cdot)$ is itself

Consider a substructure $\mathcal{M} \subseteq \mathcal{N} = (\mathbb{N}; 0, 1, +, \cdot)$. Prove that $\mathcal{M} = \mathcal{N}$. EDIT: This result seems intuitively easy, but I'm having trouble ...
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An example of a formula with infinite Morley Rank

Given a Theory and a Model, you can define the Morley Rank of formulas with parameters from the model. I'd like you to give me an example of a formula (with theory and model) with infinite Morley ...
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A fragment of Exercise 1.3.4 in _Shorter Model Theory_ by Hodges

The following is what I believe is necessary to solve Exercise 1.3.4 in Shorter Model Theory by Hodges. Given two structure $\mathcal {A, B}$ of the same signature $\tau$, a set $S$ of generators of ...
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O-minimal Theories with Non-Dense Order Type

In this paper, Knight, Pillay, and Steinhorn prove that for any O-minimal structure $\mathfrak{A}$, in which the underlying order types is dense, and if $\mathfrak{B} \equiv \mathfrak{A}$, then ...
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Perron-Frobenius theorem for real closed fields via model theory

The Perron-Frobenius theorem states that any matrix over the reals with positive entries has at least one positive eigenvalue (and a bit more). The easiest proof that I know of runs as follows: any ...
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Murder at Hilbert's Hotel!

I'm sorry if this is a duplicate in any way. I doubt it's an original question. Due to my ignorance, it's difficult for me to search for appropriate things. Motivation. This question is inspired by ...
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Formal theories dealing with non-commutattive and non-transitive notion of equality

This question is inspired by a philosophical discussion which I don't want to bother you with. As far as I know when we use (or define) the statement "$x$ is equal to $y$" in logic and ordinary ...
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Can a model (of a general theory) be viewed as a (less general) theory?

Let me explain my question on an example. As a general axiomatic theory, consider group theory. A model for group theory is, for instance, group SO(3). But group SO(3) has its own axioms, so can we ...
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An impressive fact expressible in presburger arithmetic?

Is there something expressible in presburger arithmetic that would seem impressive to students at an undergraduate level?
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What are all kind of “metamath” good for? Can it help me here? [closed]

Those logical theories, which deals with questions that isn't really mathematics but reach mathematics more or less, often seems to be like textbooks full of definitions, plus some theorems of the ...
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Difference between type and similarity type

In usual terminology, is there a difference between the type and similarity type? Is there a general consensus for the definition of the two terms? Please suggest to me books where I can study these ...
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substructures generated by constant symbols

I am have recently started to learn about model theory, so this might be a stupid question. To learn model theory, I am reading David Marker's Model Theory. This is the situation in the proof of ...
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Is the (first order theory) of Hilbert spaces categorical?

Suppose the axioms of a Hilbert space (i.e. vector space, inner product, completeness and separability) are formulated as a first order theory. It can be shown that any infinite dimensional Hilbert ...
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Uncountable Dense Linear Orders

Is there an example of two uncountable equipollent dense linear orders without endpoints that don't satisfy the same first order properties? Or is it true that two uncountable equipollent dense linear ...
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1answer
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If no interpretations satisfy a set of formulae U, is it possible for $U\models A$?

Note: '$ \models$' denotes logical consequence, defined as If $U \models A$, then $A$ is a logical consequence of $U$, if and only if every interpretation that satisfies U also satisfies $A$, ...
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Completness in higher order logic and Interpretations

It´s known that for first order theories, it holds $\mathbf{ZFC} \vdash T \vdash \varphi \leftrightarrow T \models \varphi$. Why does this not hold in the higher order case (any simple example?)? ...
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In modal logic, why are models ordered sets?

I just started undergrad math, so I only have a fuzzy idea of what a model is. I'm learning about modal logic in one of my classes. Our text describes modal logic as operating in a model defined as an ...
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Is $\sqrt{2}$ definable in $(\mathbb{R},+,\cdot,0,1)$?

For an exercise in model theory I have to state if $\sqrt{2}$ is a definable element of the structure $\mathcal{R}=(\mathbb{R},+,\cdot,0,1)$. I expect it is not, but I haven't been able to prove this. ...
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Question from Hodges' textbook Shorter Model Theory

I'm thinkng through Section 2.3. of Hodges' textbook Shorter Model Theory, problem 7(b): "Let $L$ be a first-order language. Show (without assuming that every structure is non-empty ) that every ...
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Axiomatizability of the multiplication of a ring

The operation of ring multiplication is axiomatizable, if we allow ourselves an additional auxillary addition symbol. Just write down the ring axioms in the signature $\{*,+\}$. But could ...
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1answer
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Is division axiomatizable?

Consider a set $G$ with a group operation. We can define a division operation $a*(b^{-1})$ and call it $\operatorname{div}$. Is the class of division operations first order axiomatizable? And if so, ...
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For a complete truth-set $T$ is a countable transitive model satisfying $T$ unique?

Let $T$ be a maximal (in the sense that either $\phi \in T$ or $\phi \not \in T$ for all $\phi \in \mathcal{L}_\in$) set of sentences consistent with $ZFC$. Question For a countable transitive model ...
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You don't need to take an algebraic closure twice in model theory

This is an exercise (1.4.11) from Marker. Fix a language $\mathcal L$ and $\mathcal L$-structure $\mathcal M$. For a subset $A \subseteq M$, an element of $M$ is algebraic over $A$ if it is a member ...
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Can a binary relation on a set $S$ isomorphically embed every binary relation on $S$?

Is there any binary relation $R$ on a non-empty set $S$ such that $R$ isomorphically embeds every binary relation on $S$? (By "$R$ isomorphically embeds $Q$" I mean: there is a one-to-one function ...
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Prove that $\Pi_{i \in I} \mathbf{A}_i \cong \Pi_{j \in J} (\Pi_{i \in I_j} \mathbf{A}_i)$ where $\{I_j \ | \ j \in J\}$ is a partition of I.

My problem is following: prove that $\Pi_{i \in I} \mathbf{A}_i \cong \Pi_{j \in J} (\Pi_{i \in I_j} \mathbf{A}_i)$, where $\langle \mathbf{A}_i \ | \ i \in I \rangle$ is an indexed set of similar ...
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Show that every element in class $\mathcal{K}$ have at most $n$ elements

Suppose that $\mathcal{K}$ is a class of finite structure of language $\mathcal{L}$. If $\mathcal{K}$ is axiomatizable then prove that exist $n$ such that every structure from $\mathcal{K}$ have at ...
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Why isn't there a first-order theory of well order?

Problem 1.4.1 of Model Theory by Chang and Keisler asks, Is there a theory of well order in the first-order language $\{\leq\}$? I'm pretty sure the answer is no, since well order is a property ...
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PA can define 6's multiplier?

Let set A be : {6, 36, 216, 1296 .....} i.e. A={ $6^k$} where $k \in \mathbb{n} $ In the Model PA, can PA define set A? I know PA can define set { $2^k$} and set { $3^k$}. However what about { ...
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Types realized in ultrapowers consisting of definable functions

Let $\mathcal{M}$ be a nonstandard model of arithmetic and let $M$ be its universe. Let $U$ be a nonprincipal ultrafilter over $M$ and let $\mathcal{N}$ be the ultrapower $\mathcal{M}^M / U$. Let $F$ ...
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Difference between completeness and categoricity

I have problems understanding the difference between a categorical theory and a complete theory. My intuition says that every valid complete theory must be categorical. Is it true? Clarification: by ...
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Indiscernible subsequences

Work in a saturated model $\cal U$ of sufficiently large cardinality. Are there assumptions on $\kappa<|\cal U|$ that guarantee that any sequence $\langle a_i:i<\kappa\rangle$ has an ...
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1answer
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Adding unary relation symbol within complete theory

I try to prove following problem: Let $T$ be a complete theory over countable language, then $T$ has a model $\mathfrak{A}$ with cardinality $\le 2^{\aleph_0}$ such that for each $\mathfrak{B}\models ...
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2answers
69 views

Is $Th(\mathbb{Z}[x])$ uncountably categorical?

Consider $T=Th(\mathbb{Z}[x])$ in the language $L = \{0,1,+,\times,deg(), \circ\}$ where $0,1,+$ and $\times$ have their usual interpretations, $deg()$ is a unary function symbol which gives the ...
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Are ideals necessarily definable?

Consider the first-order language of rings. Let $R$ be a ring and $I \subseteq R$ be an ideal. Is $I$ necessarily $\emptyset$-definable? If not, what if we allow parameters from $R$?