Satisfiable Set Union Tautology is Satisfiable
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Theorem
Let $\LL$ be a logical language.
Let $\mathscr M$ be a formal semantics for $\LL$.
Let $\FF$ be an $\mathscr M$-satisfiable set of formulas from $\LL$.
Let $\phi$ be a tautology for $\mathscr M$.
Then $\FF \cup \set \phi$ is also $\mathscr M$-satisfiable.
Proof
Since $\FF$ is $\mathscr M$-satisfiable, there exists some model $\MM$ of $\FF$:
- $\MM \models_{\mathscr M} \FF$
Since $\psi$ is a tautology, also:
- $\MM \models_{\mathscr M} \psi$
Therefore, we conclude that:
- $\MM \models_{\mathscr M} \FF \cup \set \phi$
that is, $\FF \cup \set \phi$ is satisfiable.
Sources
- 2012: M. Ben-Ari: Mathematical Logic for Computer Science (3rd ed.) ... (previous) ... (next): $\S 2.5.2$: Theorem $2.45$
- 2012: M. Ben-Ari: Mathematical Logic for Computer Science (3rd ed.) ... (previous) ... (next): $\S 2.10$: Exercise $2.15$