I am trying to prove this irregular induction theorem that would help prove a recursion theorem I am working on.
Can you help? Here is the theorem:
$\forall X (\forall x \in \mathbb{N} (\langle x,0 \rangle \in X \land \forall y \in \mathbb{N} (\langle x,S(y) \rangle \in X)) \rightarrow \mathbb{N} \times \mathbb{N} \subseteq X)$
I don't know if it is proveable but it seems intuitively correct. Using the second-order I think I can prove it but I want to try first order first.
Here is my work before I got stuck:
$\forall x \in \mathbb{N} (\langle x,0 \rangle \in X \land \forall y \in \mathbb{N} (\langle x,S(y) \rangle \in X))$
Premise$\forall x (x \in \mathbb{N} \rightarrow (\langle x,0 \rangle \in X \land \forall y \in \mathbb{N} (\langle x,S(y) \rangle \in X$))
Notation (1)$x \in \mathbb{N}$
Premise$x \in \mathbb{N} \rightarrow (\langle x,0 \rangle \in X \land \forall y \in \mathbb{N} (\langle x,S(y) \rangle \in X$)
Universal Instantiation $x$/$x$ (2)$\langle x,0 \rangle \in X \land \forall y \in \mathbb{N} \langle x,S(y) \rangle \in X$
Modus Ponendo Ponens (4)(3)$\forall y \in \mathbb{N} \langle x,S(y) \rangle \in X$
Simplification (5)$\forall y (y \in \mathbb{N} \rightarrow \langle x,S(y) \rangle \in X)$
Notation (6)$y \in \mathbb{N}$
Premise$y \in \mathbb{N} \rightarrow \langle x,S(y) \rangle \in X$
Universal Instantiation $y$/$y$ (8)$\langle x,S(y) \rangle \in X$
Modus Ponendo Ponens (9)(8)
I can only deduce that $\mathbb{N}\times\mathbb{N}\subset\text{dom}(X)\times\text{im}(X)$ but since $\text{dom}(X)\times\text{im}(X)$ is not necessarily a subset of $X$ it lead nowhere.