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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Is T an infinity spectrum whenever T is a spectrum?

Definitions: For a given first order sentence $\phi$ define $\text{spectrum}(\phi)$ to be the set of all cardinalities of the finite models of $\phi$. A set $S\subseteq\mathbb N_+$ is said to be a ...
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+150

Surjective exponentials for algebraically closed fields

The existence of the exponential on $\mathbb{C}$ has a very basic, yet very strong consequence : $(\mathbb{C}^*,\cdot)$ is a quotient of $(\mathbb{C},+)$. This question is concerned with fields $K$ ...
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17 views

Infinite Spectrum Problem

Let us work in a class theory like NBG. For a given first order sentence $\phi$ define $\infty\text{-spectrum}(\phi)$ to be the class of all cardinal numbers $\kappa$ for which there is a model $M$ ...
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21 views

List of primes and compactness

I'm working on the following problem: Let $p_0,p_1,...$ be a list of the prime numbers in increasing order. Show that for any set $X\subseteq\mathbb{N}$, there is a model of Th($\mathbb{N})$ which ...
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81 views

Ultraproduct with no long descending sequence

I have a countably infinite well-ordered structure $M$ (over a countable language if it helps), and an uncountable regular cardinal $κ$, and I wanted to construct an elementarily equivalent structure ...
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30 views

Ordinal pair $(α,β)$ such that $α<β$ and $Th(α,<) = Th(β,<)$

A number of weeks ago I was thinking of finding an example of a complete countable theory with only one binary predicate that is not $ω$-categorical. I later realized that $Th(\mathbb{Z},<)$ works, ...
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Do 'nice' first order logics have universal models?

A first-order logic is interpreted in a model where sentences of the logic can be said to be true or false. There may be more than one model, and we can identify morphisms between models. Do we have ...
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18 views

Are isomorphisms between finitely generated substructures determined on a set of generators?

Let $K$ be a structure, $\varphi: A \to B$ be an isomorphism of finitely generated substructures of $K$. Let $a_0,\dots,a_n$ be generators of $A$. Do the images $\varphi(a_i)$ of the generators $a_i$ ...
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Two definitions of strong homomorphism

We say that $f: A \to B$ is a homomorphism iff it preserves the operations and relations of the structure: $f(o^A(\bar{a})) = o^B(f(\bar{a}))$ where $a ∈ A^{ar(o)}$, $o$ any function symbol from the ...
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Medium-strong (graph) homomorphisms

Weak (graph) homomorphisms are mappings $f: V(G) \rightarrow V(G')$ such that the images of connected nodes $x,y$ (in the source graph) are connected: $$R(x,y) \rightarrow R(f(x),f(y)) = R(x',y')$$ ...
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why can't this proof of Löwenheim-Skolem Theorem be shorter?

An algebraic introduction to mathematical logic page 46 has the following: the proof continues on, but it seems to me we can stop here. Every consistent theory has a model, and we've just proven ...
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What are non-categorical theories about?

With theories that are categorical, it seems like you could say that the theory is about collections of objects (numbers, points, etc.) with a certain structure (the structure the standard models ...
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Modelltheory, validity of a formula

I have a question to the following task: $M=\{1,2,3\}$ and $R=\{(1,2),(1,3)\}$ Let $\mathcal{L}$ be a first order language with a binary relationsymbol $\overline{R}$ so, that $\mathcal{M}=(M,R) is ...
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61 views

Profane Model Theory, sacred Proof Theory

Dirck van Dalen starts the Preface to his Logic and Structure with the following words: "Logic appears in a ‘sacred’ and in a ‘profane’ form; the sacred form is dominant in proof theory, the ...
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Every $\alpha < \kappa^+$ can be embedded in any interval of $\kappa^{<\omega}$.

Let $\kappa$ be a cardinal. I want to show that every ordinal $\alpha < \kappa^+$ can be embedded (as an order) in any interval of $\kappa^{<\omega}$, ordered lexicographically. This is exactly ...
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2answers
26 views

Definition of an ideal in a L-language

Let $\mathcal{L}_\text{ring}=\{0,1,+, \cdot, I\}$ where $0,1$ are constants, $+, \cdot$ are binary function symbols and $I$ is an unary relation symbol. Give $\mathcal{L}$-formulas which ...
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Find a $\mathcal{L}$-formula to fullfill a condition

Let $\mathcal{L} = \{f,g\}$ where $f$ is binary and $g$ unary. Consider the $\mathcal{L}$-structure $\mathfrak{M}$ with underlying set $\mathbb{R}$ and $f^{\mathfrak{M}}$ is the default ...
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Does equivalence of algebraic categories imply bi-interpratibility of their theories?

By an algebraic theory $\mathcal{T}$ I mean any category with finite products such that the objects are given by all finite powers of some object $X$. Let $Alg\mathcal{T}$ be the concrete category of ...
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Given a structure S, can we always find a system of axioms whose valid statements are exactly those that hold in S? [closed]

If not, are there any results that have proven that this is impossible, or at least, that such a procedure would be non-effective? Also, are any examples of such structures and examples known? I ...
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definable subset of the additive theory of integers

I strongly suspect that the subset {-1,1} of (Z,+) is not definable, despite being fixed under all automorphisms of the structure. However, I can't seem to be able find a proof. Does anyone have any ...
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98 views

Prove or disprove wether the sentence $\exists x\forall y Q(x,y)\to \forall y\exists x Q(x,y)$ is logically true

I got stuck at this problem for some hours: Determine whether the first-order sentence $\exists x\forall y Q(x,y)\to \forall y\exists x Q(x,y)$ is logically true, where $Q$ is a 2-ary predicate ...
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48 views

Let $h: A \to B$ be a weak homomorphism. Is h$[A]$ a substructure of $B$?

A little bit more precise: let $\mathfrak{A}$ and $\mathfrak{B}$ be two structures. Define a weak homomorphism as a function $h: \mathfrak{A} \to \mathfrak{B}$ such that the folowing conditions are ...
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1answer
23 views

Application of compactness theorem: finite exponent groups

We call $G$ a torsion group if for every $g\in G$ there exists a natural $n>0$ such that $g^n=1$. We say that $G$ has finite exponent if the inverse quantification holds, i.e. there exists a ...
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Quantifier Elimination Tree

I found this example in "A Course in Model Theory", but don't seem to figure out why it is true. Let $L$ be a language having a unary predicate $P_s$ for each (finite) binary string $s \in \{0,1\}^*$ ...
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Properties of an elementary substructure

Let $M$ and $N$ be structures for a first order language $L$, with $M$ an elementary substructure of $N$. This means that $M$ is a substructure of $N$ and if $\varphi(x_1,\ldots,x_n)$ is a formula ...
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1answer
50 views

What is a universal function in model theory?

What does it mean that a function in a model is universal? Let A be the domain of a model. As I understand it, an empty function is a function that is not defined for any object in A; an empty n-ary ...
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Minimal model of ZFC without power set axiom

We know that $L$ is the minimal standard model of ZFC. The question is, what is the minimal "standard" model of ZFC$^-$ (meaning ZFC without the Power Set axiom)? This is really two questions: Is ...
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Does Second-Order Comprehension make second-order ZFC inconsistent due to Russell's Paradox?

When we do set theory, we take our first-order variables to range over all sets. But if we take our second-order variables to range over sets of sets in the range of the first-order variables, then ...
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1answer
39 views

Rigid relations and Choice

A binary relation $R$ on a set $D$ is rigid iff the unique $D → D$ bijection that fixes $R$ is the identity function. Any well-ordering is rigid, so the Well-Ordering Principle has the consequence ...
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Does no non-standard model of Peano Arithmetic make the integers a principal ideal domain?

Though I do not find a reference now, I have heard no non-standard model of Peano Arithmetic has a principal ideal domain as its ring of integers. Is that right? Is it trivial? Or is there a good ...
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Is the axiom of induction constructively verifiable for a non-standard model of Peano arithmetic?

There exist models of the natural numbers which include infinite numbers. Such models are called non-standard models of arithmetic. (Proof: by the compactness theorem, there exist models of Peano ...
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1answer
51 views

Completeness Theorem in logic and Completeness of a theory

Completeness Theorem says: $\Gamma \models \phi \longrightarrow \Gamma \vdash \phi$ And from definition of satisfaction: $\neg(\Gamma \models \phi) \longleftrightarrow \Gamma \models \neg\phi$ Now ...
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1answer
90 views

Can type theory be viewed as an alternative to model theory?

While type theory certainly has traditionally been used for different purposes than model theory, as noted in this Philosophy SE post, I wonder to what extent type theory could model model theory ...
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On proving the zero-one-law for first order logic

I'm trying to understand the proof of the zero-one-law for first order logic as provided in (Ebbinghaus-Flum, 1995). It goes as follows: Let $\tau$ be a relational signature. Let $r\in\mathbb{N}$, ...
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6answers
718 views

Does a finite first-order theory which has a model always have a finite model?

I'm curious whether this is true or not: Let T be a finite, first-order theory. If T has a model, then T has a finite model. I would assume the answer is 'yes', but I wanted to make sure I ...
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1answer
33 views

Model isomorphisms of a set of sentences

I have a question about models of a set of sentences $T$, specifically the following: Let $S=\{R\}$ where $R$ is a unary relation symbol. Let $T$ be the set of sentences that for each $n\geq 1$ ...
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How can I understand about ZFC and Gödel's Completeness theorem [closed]

English 1 ZFC could be formulated as First order logic. 2 Gödel's Completeness theorem is a theorem within ZFC. 3 I think a lot of books about set theory is implicitly assuming Gödel's ...
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Prove that there exists a sentence $\varphi$

I'm doing some excercises from the book "The Incompleteness Phenomenom" from Goldstern and Judah. I have to do this and I don't know how to start. Let $\Sigma_1 $ and $\Sigma_2$ be sets of sentences ...
4
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3answers
65 views

Undefinability of evenness in first order logic

My question is to show there is no sentence $\psi$ in a language of first order logic without any non-logical symbols such that for every finite structure $\mathcal{A}$: $$\mathcal{A} \vDash \psi \; ...
4
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2answers
70 views

Non existence of Prime models

Let $L$ be a countable language. Let $T$ be a complete $L$ theory. We know that if $T$ is small, then there is a prime model of the theory. But $\text{Th}(\mathbb{N},+,\times,0,1)$ is not small but it ...
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2answers
50 views

elementary question about definability

My understanding is that an object, m, from the domain of a model, M, is definable by a formula, F(x), just in case M |= (Vx)[F(x) <----> x = m]. However, this assumes that there is a name for the ...
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83 views

Gödel's incompleteness theorem applys to ZFC theory

When I assume ZFC's consistency, it is impossible to prove ZFC's consistency in itself from Gödel's incompleteness theorem 2. If ZFC's consistency have done, its proof need to be done in stronger ...
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About definition of model

In Model theory, the definition of a model is a set. Can it be a proper class? ZFC has a model and maybe some models is a proper class. Definition of a model needs to include a proper class. Is it ...
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In relatively simple words: What could be a model of $\sf {ZFC}$?

An $\text{interpretation}$ of a theory consists of A domain of discourse $\mathcal U$, usually required to be non-empty. For every constant symbol, an element of $\mathcal U$ as its ...
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What is the importance of “variety of algebras” in Universal Algebra?

Given an algebraic category, Birkhoff's Variety Theorem gives a categorical characterization of the full subcategories whose object-class forms a variety (i.e. can be defined by equations in the sense ...
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Affine space over $\mathbb {Q}$ prime model, and dimension.

For homework, i need to prove the following: Let $M$ be a structure with universe $\mathbb {Q}$ of the language that cosiest only of function symbols $f_{\bar {\alpha}}(\bar{x})$ for all $\bar ...
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101 views

absoluteness and and transitivity

I'm early in my reading about absoluteness, but one thing has me stuck, so I thought I'd ask. One reason absoluteness seems to matter is that we feel confident that we know what we're talking about ...
5
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1answer
108 views

Limits of finite structures - first order logic

Assume that $\mathcal{C}=\{M_i:i\in I\}$ is an infinite collection of different finite $\mathcal{L}$-structures in a first-order language $\mathcal{L}$. The question is: What kind of infinite ...
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25 views

Lemma (?) on syntactically saturated maximal sets of sentences and existential sentences

Is there a proof of the following claim? $$\not \vdash_{{\rm FS}(L,M)} \exists x ~\alpha \implies [\alpha][x/c] \tag{T}$$ where no variable other than $x$ occurs free, $c$ is a name, ${\rm FS}$ ...
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Algebraic numbers in real closed fields

I have been looking at the discussion of real closed fields in Appendix B of Marker's Model Theory:an Introduction. I am baffled by what it says about the uniqueness of real closures. I have no ...