Equivalences are Interderivable/Proof 1
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Theorem
If two propositional formulas are interderivable, they are equivalent:
- $\paren {p \dashv \vdash q} \dashv \vdash \paren {p \iff q}$
Proof
The result follows directly from the truth table for the biconditional:
$\begin{array}{|cc||ccc|} \hline p & q & p & \iff & q \\ \hline \F & \F & \F & \T & \F \\ \F & \T & \F & \F & \T \\ \T & \F & \T & \F & \F \\ \T & \T & \T & \T & \T \\ \hline \end{array}$
By inspection, it is seen that $\map \MM {p \iff q} = \T$ if and only if $\map \MM p = \map \MM q$.
$\blacksquare$
Sources
- 1980: D.J. O'Connor and Betty Powell: Elementary Logic ... (previous) ... (next): $\S \text{II}$: The Logic of Statements $(2): \ 6$: Using logical equivalences