Is my theorem wrong? If so where did I mess up?
The following first-order theorem doesn't feel right to me.
$$(\forall x Qx \rightarrow P)\leftrightarrow\exists x (Qx \rightarrow P)$$
(Where $x$ is not free in $P$)
Particularly:
$\exists x (Qx \rightarrow P)\rightarrow(\forall x Qx \rightarrow P)$
I am trying to prove first-order identities to be able to make more effective proofs in the future for my personal study.
I produced the following theorems using rules of inference.
I also used two theorems, which I mention after the main proof.
Theorem 1.1
- $\forall x Qx \rightarrow P$
premise - $\neg \forall x Qx \lor P$
material implication (1) - $\exists x \neg Qx \lor P$
theorem A (2) - $\exists x (\neg Qx \lor P)$
theorem B (3) - $\exists x (Qx \rightarrow P)$ material implication (4)
- $(\forall x Qx \rightarrow P)\rightarrow\exists x (Qx \rightarrow P)$
deduction theorem (1)(5)
Theorem 1.2
- $\exists x (Qx \rightarrow P)$
premise - $\exists x (\neg Qx \lor P)$
material implication (1) - $\exists x \neg Qx \lor P$
theorem B (2) - $\neg \forall x Qx \lor P$
theorem A (3) - $\forall x Qx \rightarrow P$
material implication (4) - $\exists x (Qx \rightarrow P)\rightarrow(\forall x Qx \rightarrow P)$
deduction theorem (1)(5)
Theorem 1.3
- $(\forall x Qx \rightarrow P)\rightarrow\exists x (Qx \rightarrow P)$
theorem 1.1 - $\exists x (Qx \rightarrow P)\rightarrow(\forall x Qx \rightarrow P)$
theorem 1.2 - $(\forall x Qx \rightarrow P)\leftrightarrow\exists x (Qx \rightarrow P)$
biconditional introduction (1)(2)
Theorems A and B are mentioned below for reference:
Theorem A: $$\neg\forall xPx\leftrightarrow \exists x\neg Px$$
Theorem B $$P\lor\exists xQx\leftrightarrow \exists x (P \lor Qx)$$
I will only show my proof of theorem B, but if requested I will show my proof of theorem A.
Theorem B.1
- $P \lor \exists x Qx$
premise - $\exists x Qx$
premise - $Qx$
existential instantiation (2) - $P\lor Qx$
addition (right) (3) - $\exists x (P \lor Qx)$
existential generalization (4) - $\exists x Qx \rightarrow \exists x (P \lor Qx)$
deduction theorem (2)(5) - $P$
premise - $P \lor Qx$
addition (left) (7) - $\exists x (P \lor Qx)$
existential generalization (8) - $P \rightarrow \exists x (P \lor Qx)$
deduction theorem (7)(9) - $\exists x (P \lor Qx)\lor\exists x (P \lor Qx)$
constructive dilemma (10)(6)(1) - $\exists x (P \lor Qx)$
idempotency of disjunction (11) - $P\lor\exists xQx\rightarrow \exists x (P \lor Qx)$
deduction theorem (1)(12)
Theorem B.2
- $\exists x (P \lor Qx)$
premise - $P \lor Qx$ existential instantiation
- $P$
premise - $P \lor \exists x Qx$
addition (left) - $P \rightarrow P \lor \exists x Qx$
deduction theorem (3)(4) - $Qx$
premise - $\exists x Qx$
existential generalization - $P \lor \exists x Qx$
addition (right) - $Qx \rightarrow P \lor \exists x Qx$
deduction theorem (6)(9) - $(P\lor \exists xQx)\lor(P\lor \exists xQx)$
constructive dilemma (5)(9)(2) - $P\lor \exists xQx$
idempotency of disjunction (10) - $\exists x (P \lor Qx) \rightarrow P \lor \exists x Qx$
deduction theorem (1)(12)
Theorem B
- $P\lor\exists xQx\rightarrow \exists x (P \lor Qx)$
theorem B.1 - $\exists x (P \lor Qx) \rightarrow P \lor \exists x Qx$ theorem B.2
- $P\lor\exists xQx\leftrightarrow \exists x (P \lor Qx)$ biconditional introduction (1)(2)