I wonder what should be done with the tag. Some time ago it has been proposed to be deleted.
A possible worthwhile use of the tag could be using it for questions about the logic behind a proof by contradiction (e.g. this or this), instead of questions about a particular proof by contradiction. But perhaps that would make it a very localised tag.

Shall we delete ?

I have suggested burninating here.

  • $\begingroup$ At least renaming it to proof-by-contradiction IMO, if that's what the tag is about. Before reading the tag excerpt I thought it was a subtag of fake-proofs. $\endgroup$ – Najib Idrissi Apr 6 '15 at 9:12
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    $\begingroup$ The tag is useless. Proof. Assume towards contradiction that it is useful, but this is a contradiction since the contradiction tag is useless. QED. Drop mic. $\endgroup$ – Asaf Karagila Apr 6 '15 at 9:12
  • $\begingroup$ @AsafKaragila You assumed both the result and its negation, but you may only assume the negation in a proof by contradiction. $\endgroup$ – PyRulez Apr 17 '15 at 1:42
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    $\begingroup$ @PyRulez: He is begging the question, but that's doesn't mean the proof is incorrect, just that it is not useful since the result was already known a priori. For example, given the parallel postulate as an axiom I can prove the parallel postulate using contradiction. $\endgroup$ – Fengyang Wang Apr 17 '15 at 16:13

I suggest using (proof-strategy) as the preferred synonym.

I don't think every nuance of a topic has to have a tag of its own; proof-strategy is quite narrow enough. There's no great need for a proof-by-contradiction tag.

  • $\begingroup$ ... and renaming to proof-by-contradiction as suggested by Najib Idrissi. $\endgroup$ – punctured dusk Apr 17 '15 at 7:21
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    $\begingroup$ Though it should be noted that not all contradiction questions fit within proof-strategy, so perhaps we should first re-tag some of them. $\endgroup$ – punctured dusk Apr 17 '15 at 7:24
  • $\begingroup$ I can see a notation question, which only really needs a notation tag (which it has already). $\endgroup$ – Joffan Apr 18 '15 at 14:38
  • $\begingroup$ I think that before adding synonyms for that tag, the usage of the tag (proof-strategy) should be clarified. It is rather unclear and often users use this tag for any question related to proof. See, for example, this post and other posts linked to it for some discussion of this tag. $\endgroup$ – Martin Sleziak Apr 19 '15 at 6:56

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