Hilbert Proof System Instance 2 Independence Results/Independence of A1

Theorem
Let $\mathscr H_2$ be Instance 2 of the Hilbert proof systems.

Then:

Axiom $(A1)$ is independent from $(A2)$, $(A3)$, $(A4)$.

Proof
Denote with $\mathscr H_2 - (A1)$ the proof system resulting from $\mathscr H_2$ by removing axiom $(A1)$.

Consider the constructed semantics $\mathscr C_2$.

We will prove that:


 * $\mathscr H_2 - (A1)$ is sound for $\mathscr C_2$;
 * Axiom $(A1)$ is not a tautology in $\mathscr C_2$

which leads to the conclusion that $(A1)$ is not a theorem of $\mathscr H_2 - (A1)$.

Soundness of $\mathscr H_2 - (A1)$ for $\mathscr C_2$
Starting with the axioms:

Next it needs to be shown that the rules of inference of $\mathscr H_2$ preserve $\mathscr C_2$-tautologies.

Rule $RST \, 1$: Rule of Uniform Substitution
By definition, any WFF is assigned a value $0$, $1$ or $2$.

Thus, in applying Rule $RST \, 1$, we are introducing $0$, $1$ or $2$ in the position of a propositional variable.

But all possibilities of assignments of $0$s, $1$s and $2$s to such propositional variables were shown not to affect the resulting value $0$ of the axioms.

Hence Rule $RST \, 1$ preserves $\mathscr C_2$-tautologies.

Rule $RST \, 2$: Rule of Substitution by Definition
Because the definition of $\mathscr C_1$ was given in terms of Rule $RST \, 2$, it cannot affect any of its results.

Rule $RST \, 3$: Rule of Detachment
Suppose $\mathbf A$ and $\mathbf A \implies \mathbf B$ both take value $0$.

Then using Rule $RST \, 2$, definition $(2)$, we get:


 * $\neg \mathbf A \lor \mathbf B$

taking value $0$ by assumption.

But $\neg \mathbf A$ takes value $1$ by definition of $\neg$.

So from the definition of $\lor$ it must be that $\mathbf B$ takes value $0$.

Hence Rule $RST \, 3$ also produces only WFFs of value $0$.

Rule $RST \, 4$: Rule of Adjunction
Suppose $\mathbf A$ and $\mathbf B$ take value $0$.

Then:

proving that Rule $RST \, 4$ also produces only $0$s from $0$s.

Hence $\mathscr H_2$ is sound for $\mathscr C_2$.

$(A1)$ is not a $\mathscr C_2$-tautology
Recall axiom $(A1)$, the Rule of Idempotence:


 * $(p \lor p) \implies p$

Under $\mathscr C_2$, we apply a single definitional abbreviation and have the following:


 * $\begin{array}{|cccc|c|c|} \hline

\neg & (p & \lor & p) & \lor & p \\ \hline 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 1 & 1 & 0 & 1 \\ 1 & 2 & 0 & 2 & 2 & 2 \\ \hline \end{array}$

Hence according to the definition of $\mathscr C_2$, $(A1)$ is not a tautology.

Therefore $(A1)$ is independent from $(A2)$, $(A3)$, $(A4)$.