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sav08:substitution_theorems_for_propositional_logic [2008/03/11 14:57] vkuncak |
sav08:substitution_theorems_for_propositional_logic [2008/03/11 16:11] vkuncak |
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Called metatheorems, because they are not formulas //within// the logic, but formulas //about// the logic. | Called metatheorems, because they are not formulas //within// the logic, but formulas //about// the logic. | ||
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+ | From semantics we can prove the following lemma by induction. | ||
+ | |||
+ | Lemma: if $I_1(p) = I_2(p)$ for every $p \in FV(F)$, then $e(F)(I_1)=e(F)(I_2)$. | ||
===== Substitutions ===== | ===== Substitutions ===== | ||
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++++ | ++++ | ||
- | Corrollary: if $I_1(p) = I_2(p)$ for every $p \in FV(F)$, then $e(F)(I_1)=e(F)(I_2)$. | + | Corollary (tautology instances): if $\models F$, then $\models (subst(\sigma)(F))$ for every variable substitution $\sigma$. |
- | Corollary: if $\models F$, then $\models (subst(\sigma)(F))$ for every variable substitution $\sigma$. | + | We say that two formulas are equivalent if $\models (F \leftrightarrow G)$. |
+ | **Lemma**: If $\models (F \leftrightarrow G)$ then for every interpretation $I$ we have $e(F)(I) = e(G)(I)$. | ||
- | Corollary: if $\models (F \leftrightarrow G)$ and $\sigma$ is a variable substitution, then | + | From the tautology instances Corrolary we obtain. |
- | \[ | + | |
- | \models (subst(\sigma)(F) \leftrightarrow subst(\sigma)(G) | + | |
- | \] | + | |
- | ++Does the theorem hold if $\sigma$ is not variable substitution?|No.++ | + | Corollary: if $\models (F \leftrightarrow G)$ and $\sigma$ is a variable substitution, then $\models (subst(\sigma)(F) \leftrightarrow subst(\sigma)(G))$. |
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- | We say that two formulas are equivalent if $\models (F \leftrightarrow G)$. | + | |
- | + | ||
- | **Lemma**: If $\models (F \leftrightarrow G)$ then for every interpretation $I$ we have $e(F)(I) = e(G)(I)$. | + | |
+ | Does the theorem hold if $\sigma$ is ++not a variable substitution?|No, because a general substitution could produce an arbitrary formula.++ | ||
This theorem was about transforming formulas from the outside, ignoring the structure of certain subformulas. | This theorem was about transforming formulas from the outside, ignoring the structure of certain subformulas. | ||
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**Theorem on Substituting Equivalent Subformulas**: if $\sigma$ is equivalence-preserving, then for every formula $F$ we have $\models (F \leftrightarrow (subst(\sigma)(F))$. | **Theorem on Substituting Equivalent Subformulas**: if $\sigma$ is equivalence-preserving, then for every formula $F$ we have $\models (F \leftrightarrow (subst(\sigma)(F))$. | ||
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