Soft question: Unconventional proofs I'm not sure if I understood it correctly, but one of my professors told us that one theorem was proved this way: A mathematician assumed the truth of the Riemann hypothesis and was able to prove a certain mathematical statement. Then a second mathematician assumed the negation of the Riemann hypothesis and was also able to prove the same statement. These two proofs prove that the statement is indeed true. (Does anybody know what this theorem is?)

Another example of such unconventional proof is the proof of the Fermat's last theorem for $n=5$. As I understand it, Sophie Germain showed that if ever there is a solution, one of the integers must be divisible by 5. Dirichlet then proved that if such a solution exists, then the number divisible by 5 must be odd. In the same year, Legendre proved that if such a solution exists, then the number divisible by 5 must be even. Since there are no integers that are simultaneously odd and even, no solution exists.

I also read somewhere that an unconventional way of showing that a set is nonempty is to show that its cardinality is odd (since if the cardinality is odd, it can't be zero).

Do you know of any other very interesting and unconventional proofs that are relatively easy to understand?
One example of a proof along unconventional lines is the following proof that there exist irrational numbers $a$ and $b$ such that $a^b$ is rational.

$\sqrt{2}^\sqrt{2}$ either is, or is not, rational.  If it's rational, we're done.  If not, consider 

$(\sqrt{2}^\sqrt{2})^\sqrt{2} = \sqrt{2}^{\sqrt{2} \cdot \sqrt{2}} = \sqrt{2}^2 = 2$

which is rational.  So either way, we have an example of irrational numbers $a$ and $b$ Such that $a^b$ is rational, but we have no clue which is the correct example.

Frankly, it's simpler just to consider $e^{\ln 2}$, but it's still a nice, if highly frustrating, proof.
There are lots of cool examples of unconventional proofs. The criteria for a proof is historically dependent. Very little of the work by Euclid would count as a proof in a modern mathematical journal (same goes for Euler, actually), but what they did was sufficiently rigorous for their time.

So if you look at historical proofs many will strike you as unconventional. For example, look at the Ancient Greek “proofs by exhaustion.” I’d also recommend looking at the original formulation of the solution to the Königsberg bridge problem, which started the field of graph theory.