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Every year my student's math club organizes a "proof marathon", where we present multiple proofs for a single theorem. For instance, last edition we did the AM-GM inequality with geometric, algebraic, analytic... proofs, and even one "proof" based on the laws of thermodynamics.

A list of topics we already did:

  • Euclid's theorem (the infinitude of the primes)
  • The Pythagorean theorem
  • The divergence of the harmonic series
  • The AM-GM inequality

For all of these topics, we were able to find at least 10 short proofs with lots of variety.

Some other subjects I'm considering:

  • The irrationality of $\sqrt{2}$
  • Euler's polyhedra formula
  • Fermat's little theorem

Which other theorems or results lend themselves to such a proof marathon? We're looking for easy-to-understand theorems with several short and variated proofs.

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Formula for the sum of the first $n$ positive integers: trans4mind.com/personal_development/mathematics/series/…. See also the beautiful visual proof that's the top answer at mathoverflow.net/questions/8846/proofs-without-words and read all the comments to find where the diagram first appeared. –  KCd Jul 20 at 9:15
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This probably should be a community wiki, since it does not have a single correct answer. –  RBarryYoung Jul 20 at 13:53

5 Answers 5

Several proofs that the group $(\mathbf Z/(p))^\times$ is cyclic: http://www.math.uconn.edu/~kconrad/blurbs/grouptheory/cyclicFp.pdf.

Several proofs of the evaluation of the Gaussian integral from probability theory: http://www.math.uconn.edu/~kconrad/blurbs/analysis/gaussianintegral.pdf.

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I really like the Third Proof: Bounding with the Maximal Order. It also seems to work for any multiplicative group (group of units) of a field. –  Prism Aug 13 at 3:10

I think I have seen many fundamentally different proofs of the Fundamental Theorem of Algebra, some of them easy to understand. And now I see there is a math.stack question on this here.

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Uncountability of $\mathbb R$ (and countability of $\mathbb Q$, $\mathbb Q^2$, $\mathbb Q(\sqrt 2)$...)

The fact that Euler characteristic of a triangulated object does not depend on the triangulation.

More sophisticated theorems:

Brower fixed point theorem

Theorem of invariance of the domain (or show that $\mathbb R^2$ is not homeomorphic to $\mathbb R^3$)

Let me add also the Jordan curve theorem (if one wants a simplyfied version one can restricto to polygonal curves)

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Is there a simple proof of the domain invariance without using homology theory? –  Peter Franek Jul 20 at 13:02
    
It depends how what you classify "simple". There are profs by standard "analysis" means. But you need some tools like approximation of contiunuous functions by polynomals etc... In te view of the OP, one could consider weak version of the statement, allowing map to be smooth or piece wise linear, depending on the level of the working group. –  user126154 Jul 20 at 13:21
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b.t.w. Here you find a nine account of analytic aspect of the theorem of invariance of domain. terrytao.wordpress.com/2011/06/13/… –  user126154 Jul 20 at 13:21

Should there be anyone else interested, I've also stumbled upon sixteen proofs of the isoperimetric problem (link) and fourteen of a generalization of De Bruijn's packing theorem (link).

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