Tableau Extension Lemma
Lemma
Let $T$ be a finite propositional tableau.
Let its hypothesis set $\mathbf H$ be finite.
Then $T$ can be extended to a finished finite propositional tableau $T'$ which also has hypothesis set $\mathbf H$.
General Statement
Let $\mathbf H'$ be another finite set of WFFs.
Then there exists a finished finite propositional tableau $T'$ such that:
$(1):\quad$ the root of $T'$ is $\mathbf H \cup \mathbf H'$;
$(2):\quad$ $T$ is a rooted subtree of $T'$.
Proof
Let $\mathbf A$ be a WFF of propositional logic at a node $t$ such that:
- $\mathbf A$ is not a basic WFF;
- There is a non-contradictory branch through $t$ on which $\mathbf A$ is not used.
Such a WFF $\mathbf A$ will be called unused.
Note that a tableau is finished iff there are no unused WFFs in the tableau.
Let $u \left({T}\right)$ be the length of the longest unused WFF in $T$.
If $T$ is finished, we set $u \left({T}\right) = 0$.
Since there are only finitely many WFFs in $T$, the number $u \left({T}\right)$ must exist.
Let $R \left({n}\right)$ be the proposition that every finite propositional tableau with hypothesis set $\mathbf H$ and with $u \left({T}\right) < n$ can be extended to a finite finished tableau.
That is, $R \left({n}\right)$ is an assertion that this lemma is true whenever $u \left({T}\right) < n$.
The statement $R \left({1}\right)$ is true, because a tableau $T$ such that $u \left({T}\right) < 1$ is already finished.
So, let us assume the truth of $R \left({k}\right)$.
Suppose that $u \left({T}\right) < k+1$.
We extend $T$ to a new tableau $T'$ by using every unused WFF $\mathbf A$ in $T$ once on every non-contradictory branch through $\mathbf A$.
Each of the unused WFFs in $T$ is used in the new tableau $T'$.
Also, each new WFF which was added when forming $T'$ has a length less than $u \left({T}\right)$, because the added WFFs are always shorter than the used WFFs.
So $u \left({T'}\right) < u \left({T}\right) < k + 1$, so $u \left({T'}\right) < k$.
So, by the induction hypothesis $R \left({k}\right)$, there is a finite finished extension $T$ of $T'$.
$T$ is also a finished extension of $T$.
Hence $R \left({k+1}\right)$, and the result follows by strong induction.
$\blacksquare$
Sources
- 1996: H. Jerome Keisler and Joel Robbin: Mathematical Logic and Computability ... (previous) ... (next): $\S 1.10$: Completeness: Lemma $1.10.2$