Branch of Finite Tree is Finite

Theorem

Let $T$ be a finite rooted tree with root node $r_T$.

Let $\Gamma$ be a branch of $T$.

Then $\Gamma$ is a finite branch.

Proof

Let $\Gamma$ be a branch of a finite rooted tree $T$.

Aiming for a contradiction, suppose $\Gamma$ were an infinite branch of $T$.

By definition $\Gamma$ contains an infinite number of nodes.

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From Subset of Finite Set is Finite, it follows that $\Gamma$ has a finite number of nodes.

From this contradiction it follows that $\Gamma$ can not be an infinite branch of $T$.

Hence the result.

$\blacksquare$

Sources

• 1996: H. Jerome Keisler and Joel Robbin: Mathematical Logic and Computability ... (previous) ... (next): $\S 1.7$: Tableaus