MHB Proving (x = y) using Axioms: Basic Arithmetic Proof

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The discussion centers on the expression (x = y) leading to the equivalence (y=x) if and only if (y=y). Clarification is sought on whether the focus is on the truth of the formula or its derivability. It is noted that in standard interpretations for natural numbers, closed formulas are inherently true or false without the need for axioms. For derivability, specifying the theory, such as the theory of equality, is essential. The proof involves invoking Leibniz's law for the left-to-right direction and reflexivity for the right-to-left direction.
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Which axioms (at minimum) would have to be invoked so the following expression holds:

(x = y) ----> [(y=x) <---> (y=y)] ?

All help appreciated, am
 
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It is not clear what you mean by "hold". If you are referring to the truth of this formula in the standard interpretation for natural numbers, then no axioms are involved: any closed formula is simply either true or false. If you are referring to derivability of this formula, then you need to specify the theory from which you are deriving, e.g., theory of equality. I believe the left-to-right direction can be proved using Leibniz's law, and the right-to-left direction also requires reflexivity.
 
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