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Pigeonhole is a fundamental principle without which state of mathematics will be much different. However what examples of good mathematics has not yet been proved and cannot be proved with pigeonhole alone?

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    $\begingroup$ Pythagoras' theorem $\endgroup$ – Hagen von Eitzen Nov 1 '18 at 5:46
  • $\begingroup$ Please post with explanation. Is Euler characteristic of planar graph provable? $\endgroup$ – Turbo Nov 1 '18 at 5:50
  • $\begingroup$ @edm explicit and implicit are same if principle exists in proof. $\endgroup$ – Turbo Nov 1 '18 at 6:06
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    $\begingroup$ Establishing that a given result/technique isn't sufficient to prove a given theorem is generally extremely hard to do, mostly because precisely defining what that means is extremely subtle. For example, the reverse-mathematical approach Bjorn mentions below takes as "trivial" a large amount of non-trivial mathematics (e.g. the fundamental theorem of algebra). But there are many basic results for which the pigeonhole principle seems completely irrelevant; e.g. how would you use it to prove the Pythagorean theorem (per Hagen's comment above)? $\endgroup$ – Noah Schweber Nov 1 '18 at 14:29
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    $\begingroup$ So your starting instinct here should definitely be that the pigeonhole principle - or anything else specific, really - will not be sufficient to tackle most problems you'll come across. $\endgroup$ – Noah Schweber Nov 1 '18 at 14:30
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The infinite pigeonhole principle is equivalent to B$\Sigma^0_2$ in the sense of reverse mathematics.

This implies that many theorems such as Heine-Borel, Konig's lemma, Ramsey's theorem are not provable from this Infinite Pigeonhole Principle.

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  • $\begingroup$ Possible to be more explicit in applications? $\endgroup$ – Turbo Nov 1 '18 at 6:08
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    $\begingroup$ @Brout That's a pretty explicit list of applications. Are you familiar with the theorems listed? $\endgroup$ – Noah Schweber Nov 1 '18 at 14:26
  • $\begingroup$ @NoahSchweber Except for Ramsey's theorem No. $\endgroup$ – Turbo Nov 1 '18 at 17:28

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