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I'm looking for a reference for Exercise 6.11 from Libkin's FMT book:

Prove that an order-invariant conjunctive query is FO-definable without the order relation.

All help is appreciated.

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  • $\begingroup$ Are you really looking for a reference, or just for a proof? If you need a reference to put in a paper, I don’t know any, but I wouldn’t hesitate to directly cite the Exercise in Libkin’s book. If you need a proof of the fact, it’s fairly simple. $\endgroup$ – Emil Jeřábek Aug 22 at 13:16
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    $\begingroup$ I'm looking for the reference. I agree that the proof is quite easy (if I get it right). $\endgroup$ – Bartosz Bednarczyk Aug 22 at 14:13
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    $\begingroup$ A reference to some super-theorem from which Ex. 6.11 follows would be ok too. $\endgroup$ – Bartosz Bednarczyk Aug 22 at 14:44

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