For a while now I have wanted to dig into the details of Gage-Grayson-Hamilton work on the "curve shortening flow." I have several reasons for this:
- If I am to call myself a differential geometer, I will really have to learn some more about geometric analysis fundamentals. CSF seems like the most accessible version of a large class of geometric evolution equations which are very important. (For example, Perelman's work on the Poincaré conjecture is essentially a control over the singularities of the Ricci flow introduced by Hamilton.)
- There is this big project I never finished that would likely go easier if I knew some more about these kinds of things. Maybe this would be a good way to get back into it.
- Discretized versions of the problem seem to be a place for getting students involved in mathematics. I have tried lots of different angles for getting students involved in research, and I have learned the hard way that I need a class of problems that satisfy these requirements:
- easy to get started (low floor)
- possibility of exciting new developments (high ceiling)
- I have to know something about it.
- I am teaching differential geometry this term, and I like to take the opportunity when teaching a course to deepen my knowledge of the material in some significant way.
- I am giving a talk at an undergraduate conference pretty soon, and I want to talk about these ideas. (I gave them an abstract that would push me to learn more.)
- The stuff is just cool.
So, I have started reading the primary literature already. I will write about what I am learning in small pieces, so this is an opportunity for me to organize the ideas into a sequence of blog posts.
The list below is limited a bit by my current state of knowledge. I know more details about the beginning of the list (items 1-7), and less about the end which will likely expand quite a bit.
Here is what I think will happen:
- Fundamentals on smooth plane curves: adaptations of the Frenet-Serret apparatus to 2d; some basic formulae for computation
- convex plane curves, in particular the idea of a support function; new expressions for length and area
- some isoperimetric inequalities (following a paper by Harley Flanders). Possibly several posts.
- the idea of curve shortening flow
- Gage's isoperimetric inequality
- The evolution of the isoperimetric profile under CSF.
- The Hausdorff metric on closed sets
- CSF shrinking convex curves to "round points." Likely several posts.
- About parabolic PDEs and their solutions.
- Grayson's whisker lemma and the case of embedded non-convex curves.
- Curves on surfaces.
This project is mainly for my own benefit. I want to get this stuff into my head, and writing it really helps me. If you find it interesting, too, come along.
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