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[parent] example of a universal structure (Example)

Let $ L$ be the first order language with the binary relation $ \leq$. Consider the following sentences:

  • $ \forall x,y ((x\leq y \lor y\leq x) \land ((x \leq y \land y\leq x )\leftrightarrow x=y))$
  • $ \forall x,y,z (x \leq y \land y \leq z \rightarrow x \leq z)$
Any $ L$-structure satisfying these is called a linear order. We define the relation $ <$ so that $ x<y$ iff $ x \leq y \land x \neq y$. Now consider these sentences:
  1. $ \forall x,y ((x < y \rightarrow \exists z(x<z<y))$
  2. $ \forall x \exists y,z(y<x <z)$
A linear order that satisfies 1. is called dense. We say that a linear order that satisfies 2. is without endpoints. Let $ T$ be the theory of dense linear orders without endpoints. This is a complete theory.

We can see that $ (\mathbb{Q},\leq)$ is a model of $ T$. It is actually a rather special model.

Theorem 1   Let $ (S,\leq)$ be any finite linear order. Then $ S$ embeds in $ (\mathbb{Q},\leq)$.
Proof: By induction on $ \vert S\vert$, it is trivial for $ \vert S\vert=1$.

Suppose that the statement holds for all linear orders with cardinality less than or equal to $ n$. Let $ \vert S\vert=n+1$, then pick some $ a \in S$, let $ S^{\prime}$ be the structure induced by $ S$ on $ S\setminus {a}$. Then there is some embedding $ e$ of $ S^{\prime}$ into $ \mathbb{Q}$.

  • Now suppose $ a$ is less than every member of $ S^{\prime}$, then as $ \mathbb{Q}$ is without endpoints, there is some element $ b$ less than every element in the image of $ e$. Thus we can extend $ e$ to map $ a$ to $ b$ which is an embedding of $ S$ into $ \mathbb{Q}$.
  • We work similarly if $ a$ is greater than every element in $ S^{\prime}$.
  • If neither of the above hold then we can pick some maximum $ c_{1} \in S^{\prime}$ so that $ c_{1}<a$. Similarly we can pick some minimum $ c_{2} \in S^{\prime}$ so that $ c_{2}<a$. Now there is some $ b \in \mathbb{Q}$ with $ e(c_{1})<b<e(c_{2})$. Then extending $ e$ by mapping $ a$ to $ b$ is the required embedding. $ \square$

It is easy to extend the above result to countable structures. One views a countable structure as a the union of an increasing chain of finite substructures. The necessary embedding is the union of the embeddings of the substructures. Thus $ (\mathbb{Q},\leq)$ is universal countable linear order.

Theorem 2   $ (\mathbb{Q},\leq)$ is homogeneous.
Proof: The following type of proof is known as a back and forth argument. Let $ S_{1}$ and $ S_{2}$ be two finite substructures of $ (\mathbb{Q},\leq)$. Let $ e:S_{1} \to S_{2}$ be an isomorphism. It is easier to think of two disjoint copies $ B$ and $ C$ of $ \mathbb{Q}$ with $ S_{1}$ a substructure of $ B$ and $ S_{2}$ a substructure of $ C$.

Let $ b_{1},b_{2},\ldots$ be an enumeration of $ B \setminus S_{1}$. Let $ c_{1},c_{2},\ldots,c_{n}$ be an enumeration of $ C \setminus S_{2}$. We iterate the following two step process:

The $ i$th forth step If $ b_{i}$ is already in the domain of $ e$ then do nothing. If $ b_{i}$ is not in the domain of $ e$. Then as in proposition 1, either $ b_{i}$ is less than every element in the domain of $ e$ or greater than or it has an immediate successor and predecessor in the range of $ e$. Either way there is an element $ c$ in $ C \setminus\operatorname{range}(e)$ relative to the range of $ e$. Thus we can extend the isomorphism to include $ b_{i}$.

The $ i$th back step If $ c_{i}$ is already in the range of $ e$ then do nothing. If $ c_{i}$ is not in the domain of $ e$. Then exactly as above we can find some $ b \in B \setminus \operatorname{dom}(e)$ and extend $ e$ so that $ e(b)=c_{i}$.

After $ \omega$ stages, we have an isomorphism whose range includes every $ b_{i}$ and whose domain includes every $ c_{i}$. Thus we have an isomorphism from $ B$ to $ C$ extending $ e$. $ \square$

A similar back and forth argument shows that any countable dense linear order without endpoints is isomorphic to $ (\mathbb{Q},\leq)$ so $ T$ is $ \aleph_{0}$-categorical.



"example of a universal structure" is owned by uzeromay. [ full author list (2) | owner history (1) ]
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See Also: homogeneous, $\kappa$-categorical, differential equation, example of definable type, random graph (infinite)

Also defines:  back and forth

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Cross-references: isomorphic, similar, range, successor, domain, iterate, enumeration, disjoint, argument, type, homogeneous, universal, necessary, substructures, chain, increasing, union, countable, mapping, map, image, embedding, induced, structure, cardinality, induction, proof, finite, complete theory, dense linear orders, theory, endpoints, iff, relation, linear order, sentences, binary relation, first order language
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This is version 12 of example of a universal structure, born on 2003-01-24, modified 2007-02-08.
Object id is 3925, canonical name is ExampleOfUniversalStructure.
Accessed 5757 times total.

Classification:
AMS MSC: 03C50 (Mathematical logic and foundations :: Model theory :: Models with special properties )
 03C52 (Mathematical logic and foundations :: Model theory :: Properties of classes of models)
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