NOR is Commutative
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Theorem
Let $\downarrow$ signify the NOR operation.
Then, for any two propositions $p$ and $q$:
- $p \downarrow q \dashv \vdash q \downarrow p$
That is, NOR is commutative.
Proof 1
By the tableau method of natural deduction:
Line | Pool | Formula | Rule | Depends upon | Notes | |
---|---|---|---|---|---|---|
1 | 1 | $p \downarrow q$ | Premise | (None) | ||
2 | 1 | $\neg \paren {p \lor q}$ | Sequent Introduction | 1 | Definition of Logical NOR | |
3 | 1 | $\neg \paren {q \lor p}$ | Sequent Introduction | 2 | Disjunction is Commutative | |
4 | 1 | $q \uparrow p$ | Sequent Introduction | 3 | Definition of Logical NOR |
$\Box$
By the tableau method of natural deduction:
Line | Pool | Formula | Rule | Depends upon | Notes | |
---|---|---|---|---|---|---|
1 | 1 | $q \downarrow p$ | Premise | (None) | ||
2 | 1 | $\neg \paren {q \lor p}$ | Sequent Introduction | 1 | Definition of Logical NOR | |
3 | 1 | $\neg \paren {p \lor q}$ | Sequent Introduction | 2 | Disjunction is Commutative | |
4 | 1 | $p \downarrow q$ | Sequent Introduction | 3 | Definition of Logical NOR |
$\blacksquare$
Proof by Truth Table
Apply the Method of Truth Tables:
- $\begin{array}{|ccc||ccc|} \hline p & \downarrow & q & q & \downarrow & p \\ \hline \F & \T & \F & \F & \T & \F \\ \F & \F & \T & \T & \F & \F \\ \T & \F & \F & \F & \F & \T \\ \T & \F & \T & \T & \F & \T \\ \hline \end{array}$
As can be seen by inspection, the truth values under the main connectives match for all boolean interpretations.
$\blacksquare$
Also see
Sources
- 2012: M. Ben-Ari: Mathematical Logic for Computer Science (3rd ed.) ... (previous) ... (next): $\S 2.3.3$