Existential Quantification of Provable Arithmetic Formula is Provable
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Theorem
Let $\map \phi x$ be a WFF in the language of arithmetic with $1$ free variable.
Let $\sqbrk a$ denote the unary representation of $a$.
Suppose that, for every $n \in \N$ such that $\N \models \map \phi {x \gets \sqbrk n}$:
- $\map \phi {x \gets \sqbrk n}$
is a theorem of minimal arithmetic.
Suppose also that:
- $\N \models \exists x: \map \phi x$
Then:
- $\exists x: \map \phi x$
is a theorem of minimal arithmetic.
Proof
By definition of value of formula:
- $\exists x \in \N: \map \phi x$
Choose $x_0 \in \N$ that satisfies $\map \phi {x \gets x_0}$.
By Unary Representation of Natural Number:
- $x_0 = \sqbrk {x_0}$
Thus, by Substitution Property of Equality:
- $\map \phi {x \gets \sqbrk {x_0} }$
But then, by hypothesis:
- $\map \phi {x \gets \sqbrk {x_0} }$
is a theorem of minimal arithmetic.
The result follows from Existential Generalisation.
$\blacksquare$