Definition:Finished Set of WFFs of Propositional Logic
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Definition
Let $\Delta$ be a set of WFFs of propositional logic.
Then $\Delta$ is finished if and only if:
- $\Delta$ is not contradictory
- For each WFF $\mathbf C \in \Delta$, either $\mathbf C$ is basic or one of the following is true:
- $\mathbf C$ has the form $\neg \neg \mathbf A$ where $\mathbf A \in \Delta$
- $\mathbf C$ has the form $\paren {\mathbf A \land \mathbf B}$ where both $\mathbf A \in \Delta$ and $\mathbf B \in \Delta$
- $\mathbf C$ has the form $\neg \paren {\mathbf A \land \mathbf B}$ where either $\neg \mathbf A \in \Delta$ or $\neg \mathbf B \in \Delta$
- $\mathbf C$ has the form $\paren {\mathbf A \lor \mathbf B}$ where either $\mathbf A \in \Delta$ or $\mathbf B \in \Delta$
- $\mathbf C$ has the form $\neg \paren {\mathbf A \lor \mathbf B}$ where both $\neg \mathbf A \in \Delta$ and $\neg \mathbf B \in \Delta$
- $\mathbf C$ has the form $\paren {\mathbf A \implies \mathbf B}$ where either $\neg \mathbf A \in \Delta$ or $\mathbf B \in \Delta$
- $\mathbf C$ has the form $\neg \paren {\mathbf A \implies \mathbf B}$ where both $\mathbf A \in \Delta$ and $\neg \mathbf B \in \Delta$
- $\mathbf C$ has the form $\paren {\mathbf A \iff \mathbf B}$ where either:
- both $\mathbf A \in \Delta$ and $\mathbf B \in \Delta$, or:
- both $\neg \mathbf A \in \Delta$ and $\neg \mathbf B \in \Delta$;
- $\mathbf C$ has the form $\neg \paren {\mathbf A \iff \mathbf B}$ where either:
- both $\mathbf A \in \Delta$ and $\neg \mathbf B \in \Delta$
- or:
- both $\neg \mathbf A \in \Delta$ and $\mathbf B \in \Delta$.
Also see
- Definition:Tableau Extension Rules: note the similarity.
Sources
- 1996: H. Jerome Keisler and Joel Robbin: Mathematical Logic and Computability ... (previous) ... (next): $\S 1.9$: Finished Sets