# Extended Completeness Theorem for Propositional Tableaus and Boolean Interpretations

## Contents

## Theorem

Tableau proofs (in terms of propositional tableaus) are a strongly complete proof system for boolean interpretations.

More precisely, for every countable collection $\mathbf H$ of WFFs of propositional logic and every WFF $\mathbf A$:

- $\mathbf H \models_{\mathrm{BI}} \mathbf A$ implies $\mathbf H \vdash_{\mathrm{PT}} \mathbf A$

## Proof

Let $\mathbf A$ be a semantic consequence of $\mathbf H$ for boolean interpretations.

That is, if $v \models_{\mathrm{BI}} \mathbf H$, also $v \models_{\mathrm{BI}} \mathbf A$.

By the truth function for $\neg$, it follows that for such $v$:

- $v \not\models_{\mathrm{BI}} \neg \mathbf A$

Therefore, $\mathbf H' := \mathbf H \cup \left\{{\mathbf A}\right\}$ is unsatisfiable for boolean interpretations.

Since $\mathbf H'$ is countable, it follows from the Compactness Theorem for Boolean Interpretations that:

- Some finite $\mathbf H'' \subseteq \mathbf H'$ is unsatisfiable.

By the Tableau Extension Lemma, there exists a finite finished tableau $T$ for $\mathbf H''$.

By definition of finished tableau, every branch of $T$ is finished or contradictory.

From the Corollary to the Finished Branch Lemma, $\Phi \left[{\Gamma}\right]$ is satisfiable for any finished branch $\Gamma$.

But since $\mathbf H'' \subseteq \Phi \left[{\Gamma}\right]$, this would imply $\mathbf H''$ is also satisfiable, which is a contradiction.

It follows that every branch of $T$ is contradictory.

Since $\mathbf H'' \subseteq \mathbf H'$, replacing the hypothesis set $\mathbf H'$ of $T$ with $\mathbf H''$ yields another propositional tableau $T'$.

Since every branch of $T'$ is contradictory, $T'$ is a tableau confutation of $\mathbf H'$.

Recalling that $\mathbf H' = \mathbf H \cup \left\{{\neg\mathbf A}\right\}$, we conclude that $T'$ is a tableau proof of $\mathbf A$ from $\mathbf H$:

- $\mathbf H \vdash_{\mathrm{PT}} \mathbf A$

$\blacksquare$

## Also see

The Extended Soundness Theorem for Propositional Tableaus and Boolean Interpretations in which is proved:

- If $\mathbf H \vdash \mathbf A$, then $\mathbf H \models \mathbf A$.

## Sources

- 1996: H. Jerome Keisler and Joel Robbin:
*Mathematical Logic and Computability*... (previous) ... (next): $\S 1.10$: Completeness: Theorem $1.10.3$