Rule of Conjunction/Sequent Form/Formulation 2
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Theorem
The Rule of Conjunction can be symbolised by the sequent:
- $\vdash p \implies \paren {q \implies \paren {p \land q} }$
Proof
This proof is derived in the context of the following proof system: Instance 2 of the Hilbert-style systems.
By the tableau method:
Line | Pool | Formula | Rule | Depends upon | Notes | |
---|---|---|---|---|---|---|
1 | $\neg p \lor p$ | Law of Excluded Middle | ||||
2 | $\paren {\neg p \lor p} \implies \paren {p \lor \neg p}$ | Axiom $\text A 3$ | $\neg p \,/\, p, p \,/\, q$ | |||
3 | $p \lor \neg p$ | Rule $\text {RST} 3$ | 1, 2 | |||
4 | $\paren {\neg p \lor \neg q} \lor \neg \paren {\neg p \lor \neg q}$ | Rule $\text {RST} 1$ | 3 | $\paren {\neg p \lor \neg q} \,/\, p$ | ||
5 | $\paren {\neg p \lor \neg q} \lor \paren {p \land q}$ | Rule $\text {RST} 2 (1)$ | ||||
6 | $\paren {\paren {\neg p \lor \neg q} \lor \paren {p \land q} } \implies \paren {\neg p \lor \paren {\neg q \lor \paren {p \land q} } }$ | Rule of Association | $\neg p \,/\, p, \neg q \,/\, q, \paren {p \land q} \,/\, r$ | |||
7 | $\neg p \lor \paren {\neg q \lor \paren {p \land q} }$ | Rule $\text {RST} 3$ | 5, 6 | |||
8 | $p \implies \paren {q \implies \paren {p \land q} }$ | Rule $\text {RST} 2 (2)$ | 7 |
$\blacksquare$
Sources
- 1959: A.H. Basson and D.J. O'Connor: Introduction to Symbolic Logic (3rd ed.) ... (previous) ... (next): $\S 4.7$: The Derivation of Formulae: $D \, 11, 12$