Isochronous Curves
Variation calculus is a great lens through which to look at many physics and CS problems. It is IMO under-taught and under-utilized, especially in the machine learning discipline.<p>The framework is basically minimization in functional space (as opposed to R^n or a subset thereof, the more common case).<p>The (to me) surprising thing is that finding an extremum in functional space (i.e. infinite-dimensional space) can be reduced to solving a differential equation, via Euler–Lagrange.<p>The first problem feels intractable, while the second is "just" numerical integration.