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In general a representation theorem is — according to Wikipedia — a "theorem that states that every abstract structure with certain properties is isomorphic to a concrete structure". Wikipedia gives a list of canonical examples.

[Side remark: Interesting enough, the only genuinely set theoretical example (Mostowski's collapsing theorem) is filed under "category theory", and Dedekind's lemma which is fairly easy to grasp is missing in this list.]

I'd like to ask:

Can Gödel's completeness theorem be considered a representation theorem?

I dare to ask because

  1. Consistent logical formulae (or sets of, ie. theories) can be considered as (descriptions of) abstract structures
  2. The (set theoretical) models of such formulae (resp. theories) are concrete structures.

One important difference between Gödel's incompleteness theorem and some of the representation theorems listed at Wikipedia is, that not only in it's proofs (known to me) the concrete structures don't "come free with" the abstract structures — as most easily seen in the Dedekind case of posets — but that the proofs are non-constructive at all.

So I'd like to flank my question:

Is the Wikipedia "definition" of the notion of a representation theorem adequate or would practicing representation theorists refine it, e.g. to exclude "non-constructive" theorems.

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    Representation theory is *not* the "theory of representing concrete theories". I also fail to see any reason whatsoever for the incompleteness theorem to be a "representation theorem", if anything it is the complete opposite. It tells us that under some constraints there is no concrete model.2012-02-15
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    Please help me to fix the misunderstandings: a) I used the term "representation *theory*" only in the tag, else I talked about representation *theorems*. b) I asked about the *completeness* theorem (sic!), not about the **in** completeness theorem.2012-02-15
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    As for (a) my point was exactly aimed to the tag. Much like [logic] does not apply for everything which requires "a logical thinking" and [set-theory] has nothing to do with just any questions with sets. As for (b)... I guess that is what happens when you're hungover. However the point remains, the completeness theorem just connects syntax and semantics - it does not imply anything is isomorphic to another.2012-02-15
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    As for (a): I see your point, but I want to leave the tag, since there is no tag for "rep. theorems", and there *is* - presumably - a connection between "rep. theory" and "rep. theorem". As for (b): Maybe you are right, but for me it's discussion-worthy whether the compl. theorem *really* **just** connects syntax and semantics and whether it *really* **does not imply anything** about one thing being isomorphic to another.2012-02-15
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    There is no direct connection between [representation theory](http://en.wikipedia.org/wiki/Representation_theory) and representation theorems; this is why I approved the edit deleting the tag from the question.2012-02-15
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    @anon: Isn't this arguable? What is a "direct connection" between two notions? (BTW: I have no problem with the deletion of the tag if you find it appropriate.)2012-02-15
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    let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/2521/discussion-between-hans-stricker-and-anon)2012-02-16

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The completeness theorem of Gödel for languages of arbitrary cadinality is, in the absence of the axiom of choice, equivalent to a well know representation theorem, the Stone representation theorem that says (among other things) that every Boolean algebra is isomorphic to an algebra of sets and this is in turn equivalent to a highly nonconstructive choice principle, the Boolean prime ideal theorem. So representation theorems do not have to be constructive.

But the completeness theorem is not a representation theorem because the model one creates (if "construct" sounds too constructive for your taste) is not isomorphic to the theory. It is in some sense an extension theorem that says, informally, that any consistent description can be extended to a complete description of everything. Since the initial description is not complete, we can complete it in several ways.

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    Thank you! I definitely was not aware of the fact that Gödel's CT was equivalent to Stone's RT. Is this commonly known (and easy to proof), or do you have a reference? The second part of your answer I understand better. Should I restrict my question to categorical theories?2012-02-15
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    How crucial is "arbitrary cardinality" and "absence of AC"?2012-02-15
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    I guess it is known to many logicians. A book that makes the connection very clear is "Models and Ultraproducts" by Bell and Slomson. The "Handbook of Analysis and its Foundations" contains the proof too, and so does "The Axiom of Choice" by Jech. The relation to logic is, very roughly, that a theory can be seen as a family of sets of sentences closed under subsets and fpairwise union. If it is consistent, it doesn't contain the set of all sentences. Together, this means that a consistent theory is an ideal. Now a maximal consistent family is therefore a prime ideal.2012-02-15
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    For countable theories, the completeness theorem is provably in ZF. Essentially, you can list all propositions and inductively decide wheter the should be assigned the value true or false.2012-02-15
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    Thanks. This is really helpful. (Referring to: "I guess it is known...")2012-02-15
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    "CT is provably in ZF" = "it is provable that CT is in ZF"? **or** "CT is provable in ZF"? (I am honestly confused.)2012-02-15
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    You can prove in ZF that CT is true.2012-02-15
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    let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/2505/discussion-between-hans-stricker-and-michael-greinecker)2012-02-15
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    @Hans Stricker: Let $T$ be a consistent theory. Then a model of $T$ can be "constructed" from syntactic elements. Key words are the introduction of *Skolem functions*.2012-02-15