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I'm can't solve Exercise 2 on page 30 on Rautenberg (Chapter 1.4 - A calculus of Natural Deduction), which reads:

Augment the signature $\{\lnot, \land\}$ by $\lor$ and prove the completeness of the calculus obtained by supplementing the basic rules used so far (see at the bottom of my question) with the rules:

$(\lor1)$ $\dfrac{X \vdash \alpha}{X \vdash \alpha \lor \beta, \beta \lor \alpha}$

$(\lor2)$ $\dfrac{X,\alpha \vdash \gamma \ \mid \ X,\beta\vdash \gamma}{X,\alpha \lor \beta \vdash , \gamma}$

The solution and hints reads:

Prove that $X \vdash \alpha \lor \beta \iff X\vdash \alpha$ or $X\vdash \beta$. It is clear to me why this is needed, however I can't prove $\Rightarrow$. If you could please help. Thank-you

These are the inference rules for the system (+ the two above)

(IS) $\dfrac{}{\alpha\vdash\alpha}$

(MR) $\dfrac{X\vdash\alpha}{X'\vdash\alpha}$, if $X\subseteq$ X'

($\land$1) $\dfrac{X\vdash\alpha,\beta}{X\vdash\alpha\land\beta}$

($\land$2) $\dfrac{X\vdash\alpha\land\beta}{X\vdash\alpha,\beta}$

($\lnot$1) $\dfrac{X\vdash\alpha,\lnot\alpha}{X\vdash\beta}$

($\lnot$2) $\dfrac{X,\alpha \vdash \beta \ \mid \ X,\lnot\alpha\vdash \beta}{X\vdash \beta}$

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  • $\begingroup$ Can you write down all the inference rules of your system? $\endgroup$ – Taroccoesbrocco Aug 10 '18 at 15:23
  • $\begingroup$ Please find the inference rules at the bottom of my question. Thanks $\endgroup$ – Lorenzo Gandini Aug 10 '18 at 17:53
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You cannot prove in any classical logic: $X \vdash α \vee β$ iff $X \vdash α$ or $X \vdash β$ in general. Here is a counter example:

$$p \vee \neg p \vdash p \vee \neg p$$

But you don't have neither:

$$p \vee \neg p \nvdash p$$

$$p \vee \neg p \nvdash \neg p$$

You can argue semantically $p \vee \neg p$, as a Tautology, has two models $p=0$ and $p=1$. But the model $p=0$ prevents a consequence $p$. And the model $p=1$ prevents a consequence $\neg p$.

Edit: What Routenbergs Exercise Nr. 2 on Page 30 is about, is showing completness. So you would proceed with a consistent X and construct a maximally consistent X' (like in his Theorem 4.6), where you would indeed then have if $α \vee β \in X'$, then either $α \in X'$ or $β \in X'$.

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