MHB Infinite Natural Numbers: First-Order Logic Formula Explained

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The discussion explains that there are infinitely many natural numbers based on several foundational facts: the existence of at least one natural number, the existence of a distinct successor for each natural number, the uniqueness of successors, and the existence of a natural number (0) that is not a successor. A first-order logic formula is proposed that is satisfiable only if the domain is infinite, emphasizing the properties of the successor relation. The formula asserts that if two successors are equal, their original numbers must also be equal, and that every natural number has a successor that is not zero. This logical structure reinforces the concept of infinite natural numbers.
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There are an infinite number of natural numbers. Why is that? Well this follows from the following facts:

(i) There is at least one natural number.

(ii) For each natural number there is a distinct number which is its successor, i.e., for each number $x$ there is a distinct number $y$ such that $y$ stands in the
successor relation to $x$.

(iii) No two natural numbers have the same successor.

(iv) There is a natural number, namely 0, that is not the successor of any number.Bearing these facts in mind, what's a formula in first-order logic that that is satisfiable by a valuation only if the domain of the valuation is infinite. Contain some non-logical vocabulary in presentation of course.
 
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$$(\forall x\forall y\,(S(x)=S(y)\to x=y))\land \forall x\,S(x)\ne0$$
 
Evgeny.Makarov said:
$$(\forall x\forall y\,(S(x)=S(y)\to x=y))\land \forall x\,S(x)\ne0$$

thanks that helps
 
I was reading documentation about the soundness and completeness of logic formal systems. Consider the following $$\vdash_S \phi$$ where ##S## is the proof-system making part the formal system and ##\phi## is a wff (well formed formula) of the formal language. Note the blank on left of the turnstile symbol ##\vdash_S##, as far as I can tell it actually represents the empty set. So what does it mean ? I guess it actually means ##\phi## is a theorem of the formal system, i.e. there is a...
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