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I'm having difficulties to prove that the subset of even numbers is not first-order definable in $(\N,<)$. Any hint is welcomed!

More generally, what are usual techniques in order to prove that a subset is not FO-definable. I know one using isomorphism.

What are other methods?

I'm having difficulties to prove that the subset of even numbers is not first-order definable in $(\mathbb{N},<)$. Any hint is welcomed!

More generally, what are usual techniques in order to prove that a subset is not FO-definable. I know one using isomorphism.

What are other methods?