# Compactness Theorem/Proof using Henkin Construction

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## Theorem

Let $\LL$ be the language of predicate logic.

Let $T$ be a set of $\LL$-sentences.

Then $T$ is satisfiable if and only if $T$ is finitely satisfiable.

## Proof

By definition, $T$ is finitely satisfiable means that every finite subset of $T$ is satisfiable.

Because the direction:

- $T$ satisfiable implies $T$ finitely satisfiable

is trivial, the proof below justifies the converse:

- $T$ finitely satisfiable implies $T$ satisfiable.

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This proof actually demonstrates a stronger form of the Compactness Theorem by showing:

- If $T$ is finitely satisfiable and $\kappa$ is an infinite cardinal such that $\kappa > \size \LL$, then $T$ is satisfiable by a model whose cardinality is at most $\kappa$.

This is stronger than the original statement, since it provides extra information about the model that is claimed to exist.

The proof is said to use a Henkin Construction because it involves the construction of a model all of whose elements are interpretations of the constant symbols of some language.

Such a model was used in Leon Albert Henkin's proof of the Completeness Theorem.

This special feature of the constructed model is what allows us to control its cardinality.

First, by Extend Theory to Satisfy Witness Property, we can extend $\LL$ and $T$ to find a language $\LL^*$ of cardinality at most $\kappa$ and an $\LL^*$-theory $T^*$ such that all finitely satisfiable $\LL^*$-theories containing $T^*$ have the witness property.

Then, since finitely satisfiable theories have maximal finitely satisfiable extensions, we can find a finitely satisfiable $\LL^*$-theory $T'$ containing $T^*$ such that $T'$ contains either $\phi$ or $\neg\phi$ for each $\LL^*$-sentence $\phi$.

Note that $T'$ has the witness property since it contains $T^*$.

Finally, by Maximal Finitely Satisfiable Theory with Witness Property is Satisfiable, $T'$ has a model.

Moreover, since $\LL^*$ has cardinality at most $\kappa$ and hence has at most $\kappa$-many constant symbols, this theorem ensures that the model of $T'$ can be taken to have cardinality at most $\kappa$.

$\blacksquare$

## Sources

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- 2009: Kenneth Kunen:
*The Foundations of Mathematics*... (previous) ... (next): $\text {II}.7$ First-Order Logic Semantics: Theorem $\text {II}.7.15$