Elliptic curves for high school students
I had to give a talk to high school students about some mathematical notion: I decided to tell them something about elliptic curves, but not the usual speech about cryptography, finite fields and the group law on a cubic curve…
Instead, I talked about the perhaps less known appearances of elliptic functions as solutions of classical ODEs (even if I don’t really know much about these myself). The simplest mechanical system whose motion is governed by an elliptic curve is the pendulum: the reason for this is that the ODE which classically describes the time evolution of the angle of the pendulum is best rewritten in terms of the altitude of the pendulum: the law of energy conservation is then written as
where 0 and 2l are the extremal values of the altitude , is the highest altitude which can be reached with a given energy (even if , which corresponds to the pendulum make full rotations around its axis), and is the vertical momentum of the pendulum.
In this setting, there are classical Hamilton relations , , so the differential form turns out to be the canonical non-vanishing abelian differential on the elliptic curve. This explains why the period of the pendulum is an elliptic integral, which can be calculed by an arithmetic-geometric mean, and why the position of the pendulum at can be deduced from its position at times and by the classical secant-tangent law.
The notes for the talk (in French) are available here.