Are there any simple and straightforward proofs of this fact? I'm not really sure how to begin to approach the problem.
2026-04-05 18:25:56.1775413556
Why isn't $\leq$ definable in $(\mathbb{R};0,+,-)$?
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Hint: definable sets are preserved under automorphisms. Can you come up with an automorphism where $a\leq b$ but $f(a)>f(b)$?