# The polymath blog

## June 12, 2012

### Polymath7 research thread 1: The Hot Spots Conjecture

Filed under: hot spots,research — Terence Tao @ 8:58 pm

The previous research thread for the Polymath7 project “the Hot Spots Conjecture” is now quite full, so I am now rolling it over to a fresh thread both to summarise the progress thus far, and to make it a bit easier to catch up on the latest developments.

The objective of this project is to prove that for an acute angle triangle ABC, that

1. The second eigenvalue of the Neumann Laplacian is simple (unless ABC is equilateral); and
2. For any second eigenfunction of the Neumann Laplacian, the extremal values of this eigenfunction are only attained on the boundary of the triangle.  (Indeed, numerics suggest that the extrema are only attained at the corners of a side of maximum length.)

To describe the progress so far, it is convenient to draw the following “map” of the parameter space.  Observe that the conjecture is invariant with respect to dilation and rigid motion of the triangle, so the only relevant parameters are the three angles $\alpha,\beta,\gamma$ of the triangle.  We can thus represent any such triangle as a point $(\alpha/\pi,\beta/\pi,\gamma/\pi)$ in the region $\{ (x,y,z): x+y+z=1, x,y,z > 0 \}$.  The parameter space is then the following two-dimensional triangle:

Thus, for instance

1. A,N,P represent the degenerate obtuse triangles (with two angles zero, and one angle of 180 degrees);
2. B,F,O represent the degenerate acute isosceles triangles (with two angles 90 degrees, and one angle zero);
3. C,E,G,I,L,M represent the various permutations of the 30-60-90 right-angled triangle;
4. D,J,K represent the isosceles right-angled triangles (i.e. the 45-45-90 triangles);
5. H represents the equilateral triangle (i.e. the 60-60-60 triangle);
6. The acute triangles form the interior of the region BFO, with the edges of that region being the right-angled triangles, and the exterior being the obtuse triangles;
7. The isosceles triangles form the three line segments NF, BP, AO.  Sub-equilateral isosceles triangles (with apex angle smaller than 60 degrees) comprise the open line segments BH,FH,OH, while super-equilateral isosceles triangles (with apex angle larger than 60 degrees) comprise the complementary line segments AH, NH, PH.

Of course, one could quotient out by permutations and only work with one sixth of this diagram, such as ABH (or even BDH, if one restricted to the acute case), but I like seeing the symmetry as it makes for a nicer looking figure.

Here’s what we know so far with regards to the hot spots conjecture:

1. For obtuse or right-angled triangles (the blue shaded region in the figure), the monotonicity results of Banuelos and Burdzy show that the second claim of the hot spots conjecture is true for at least one second eigenfunction.
2. For any isosceles non-equilateral triangle, the eigenvalue bounds of Laugesen and Siudeja show that the second eigenvalue is simple (i.e. the first part of the hot spots conjecture), with the second eigenfunction being symmetric around the axis of symmetry for sub-equilateral triangles and anti-symmetric for super-equilateral triangles.
3. As a consequence of the above two facts and a reflection argument found in the previous research thread, this gives the second part of the hot spots conjecture for sub-equilateral triangles (the green line segments in the figure). In this case, the extrema only occur at the vertices.
4. For equilateral triangles (H in the figure), the eigenvalues and eigenfunctions can be computed exactly; the second eigenvalue has multiplicity two, and all eigenfunctions have extrema only at the vertices.
5. For sufficiently thin acute triangles (the purple regions in the figure), the eigenfunctions are almost parallel to the sector eigenfunction given by the zeroth Bessel function; this in particular implies that they are simple (since otherwise there would be a second eigenfunction orthogonal to the sector eigenfunction).  Also, a more complicated argument found in the previous research thread shows in this case that the extrema can only occur either at the pointiest vertex, or on the opposing side.

So, as the figure shows, there has been some progress on the problem, but there are still several regions of parameter space left to eliminate.  It may be possible to use perturbation arguments to extend validity of the hot spots conjecture beyond the known regions by some quantitative extent, and then use numerical verification to finish off the remainder.  (It appears that numerics work well for acute triangles once one has moved away from the degenerate cases B,F,O.)

The figure also suggests some possible places to focus attention on, such as:

1. Super-equilateral acute triangles (the line segments DH, GH, KH).  Here, we know the second eigenfunction is simple (and anti-symmetric).
2. Nearly equilateral triangles (the region near H).  The perturbation theory for the equilateral triangle could be non-trivial due to the repeated eigenvalue here.
3. Nearly isosceles right-angled triangles (the regions near D,G,K).  Again, the eigenfunction theory for isosceles right-angled triangles is very explicit, but this time the eigenvalue is simple and perturbation theory should be relatively straightforward.
4. Nearly 30-60-90 triangles (the regions near C,E,G,I,L,M).  Again, we have an explicit simple eigenfunction in the 30-60-90 case and an analysis should not be too difficult.

There are a number of stretching techniques (such as in the Laugesen-Siudeja paper) which are good for controlling how eigenvalues deform with respect to perturbations, and this may allow us to rigorously establish the first part of the hot spots conjecture, at least, for larger portions of the parameter space.

As for numerical verification of the second part of the conjecture, it appears that we have good finite element methods that seem to give accurate results in practice, but it remains to find a way to generate rigorous guarantees of accuracy and stability with respect to perturbations.  It may be best to focus on the super-equilateral acute isosceles case first, as there is now only one degree of freedom in the parameter space (the apex angle, which can vary between 60 and 90 degrees) and also a known anti-symmetry in the eigenfunction, both of which should cut down on the numerical work required.

I may have missed some other points in the above summary; please feel free to add your own summaries or other discussion below.

## June 9, 2012

Filed under: discussion,hot spots — Terence Tao @ 5:50 am

The “Hot spots conjecture” proposal has taken off, with 42 comments as of this time of writing.  As such, it is time to take the proposal to the next level, by starting a discussion thread (this one) to hold all the meta-mathematical discussion about the proposal (e.g. organisational issues, feedback, etc.), and also starting a wiki page to hold the various facts, strategies, and bibliography around the polymath project (which now is “officially” the Polymath7 project).

I’ve seeded the wiki with the links and references culled from the original discussion, but it was a bit of a rush job and any editing would be greatly appreciated.  From past polymath experience, these projects can get difficult to follow from the research threads alone once the discussion takes off, so the wiki becomes a crucial component of the project as it can be used to collate all the progress made so far and make it easier for people to catch up.  (If the wiki page gets more complicated, we can start shunting off some stuff into sub-pages, but I think it is at a reasonable size for now.)

One thing I see is that not everybody who has participated knows how to make latex formatting such as $\Delta u = \lambda u$ appear in their comments.  The instructions for that (as well as a “sandbox” to try out the code) are at this link.

Once the research thread gets long enough, we usually start off a new thread (with some summaries of the preceding discussion) to make it easier to keep the discussion at a manageable level of complexity; traditionally we do this at about the 100-comment mark, but of course we can alter this depending on how people are able to keep up with the thread.

## June 3, 2012

### Polymath proposal: The Hot Spots Conjecture for Acute Triangles

Filed under: hot spots,polymath proposals — Terence Tao @ 2:59 am

Chris Evans has proposed a new polymath project, namely to attack the “Hot Spots conjecture” for acute-angled triangles.   The details and motivation of this project can be found at the above link, but this blog post can serve as a place to discuss the problem (and, if the discussion takes off, to start organising a more formal polymath project around it).

## November 13, 2011

### Lipton’s Polymath Proposal: The Group Isomorphism Problem

Filed under: polymath proposals — Gil Kalai @ 10:16 am
Tags: , ,

## July 19, 2011

### Minipolymath3 project: 2011 IMO

Filed under: research — Terence Tao @ 8:00 pm

This post marks the official opening of the mini-polymath3 project to solve a problem from the 2011 IMO.  I have decided to use Q2, in part to see how the polymath format would cope with a more geometrically themed problem.

Problem 2.  Let $S$ be a finite set of at least two points in the plane. Assume that no three points of $S$ are collinear. A windmill is a process that starts with a line $\ell$ going through a single point $P \in S$. The line rotates clockwise about the pivot $P$ until the first time that the line meets some other point $Q$ belonging to $S$. This point $Q$ takes over as the new pivot, and the line now rotates clockwise about $Q$, until it next meets a point of $S$. This process continues indefinitely.
Show that we can choose a point $P$ in $S$ and a line $\ell$ going through $P$ such that the resulting windmill uses each point of $S$ as a pivot infinitely many times.
The comments to this post shall serve as the research thread for the project, in which participants are encouraged to post their thoughts and comments on the problem, even if (or especially if) they are only partially conclusive.  Participants are also encouraged to visit the discussion thread for this project, and also to visit and work on the wiki page to organise the progress made so far.
This project will follow the general polymath rules.  In particular:
1. All are welcome. Everyone (regardless of mathematical level) is welcome to participate.  Even very simple or “obvious” comments, or comments that help clarify a previous observation, can be valuable.
2. No spoilers! It is inevitable that solutions to this problem will become available on the internet very shortly.  If you are intending to participate in this project, I ask that you refrain from looking up these solutions, and that those of you have already seen a solution to the problem refrain from giving out spoilers, until at least one solution has already been obtained organically from the project.
3. Not a race. This is not intended to be a race between individuals; the purpose of the polymath experiment is to solve problems collaboratively rather than individually, by proceeding via a multitude of small observations and steps shared between all participants.   If you find yourself tempted to work out the entire problem by yourself in isolation, I would request that you refrain from revealing any solutions you obtain in this manner until after the main project has reached at least one solution on its own.
4. Update the wiki. Once the number of comments here becomes too large to easily digest at once, participants are encouraged to work on the wiki page to summarise the progress made so far, to help others get up to speed on the status of the project.
5. Metacomments go in the discussion thread. Any non-research discussions regarding the project (e.g. organisational suggestions, or commentary on the current progress) should be made at the discussion thread.
6. Be polite and constructive, and make your comments as easy to understand as possible. Bear in mind that the mathematical level and background of participants may vary widely.

Have fun!

## May 12, 2011

### Possible new polymath project

Filed under: polymath proposals — Terence Tao @ 5:56 pm

Richard Lipton has just proposed on his blog to discuss the following conjecture of Erdos as a polymath project: that there are no natural number solutions to the equation

$1^k + \ldots + (m-1)^k = m^k$

with $k \geq 2$.  Previous progress on this problem (including, in particular, a proof that any solution to this equation must have an extremely large value of $m$, and specifically that $m \geq 10^{10^9}$) can be found here.

## April 28, 2011

### Polymath wiki logo

Filed under: planning — Terence Tao @ 4:37 pm

Michael Nielsen has collected a number of possible logos for the polymath wiki and is asking for discussion on them.

## March 9, 2011

### Polymath discussion at IAS

Filed under: discussion — Gil Kalai @ 2:26 pm
Tags: , ,

In October 2010 there was a discussion about polymath projects at an event organized by the I.A.S in NYC. Tim Gowers described the endeavor and some prospects, and hopes, and Peter Sarnak responded with some concerns. An interesting discussion followed. Some of the discussion is described in the IAS Institute Letter for fall 2010 .

## February 14, 2011

### Polymath4: Referee report obtained

Filed under: finding primes,news — Terence Tao @ 11:34 am

An update on the status of the Polymath4 paper on finding primes.  I’ve received a referee report from Mathematics of Computation on the submission, which can be found here.   The referee liked the result but wanted a fair number of expository changes before he or she was willing to recommend acceptance, so the editor has asked for a revision.  I will be happy to make the relevant changes, but if there are any other changes that other participants would like to make, now would be a good time to suggest them.  (The most recent version of the paper can be found at the Subversion repository or at this link; see also the arXiv version.)

One change requested is to add a list of participants to the project.  In analogy with what we did for Polymath1, I therefore started a “signup sheet” on the wiki at

http://michaelnielsen.org/polymath1/index.php?title=Polymath4_grant_acknowledgments

for people to self-report their participation, contact information, and grant information for the project.    There is the usual problem of trying to decide who is a “main participant” of the project, and who is a “contributor” (though I think I can safely add Ernie, Harald, and myself as participants); as with Polymath1, I will leave it to each of you to self-report what level of participation you feel is appropriate.

## February 13, 2011

### Can Bourgain’s argument be usefully modified?

Filed under: Improving Roth bounds — gowers @ 6:23 pm

I’ve been feeling slightly guilty over the last few days because I’ve been thinking privately about the problem of improving the Roth bounds. However, the kinds of things I was thinking about felt somehow easier to do on my own, and my plan was always to go public if I had any idea that was a recognisable advance on the problem.

I’m sorry to say that the converse is false: I am going public, but as far as I know I haven’t made any sort of advance. Nevertheless, my musings have thrown up some questions that other people might like to comment on or think about.

Two more quick remarks before I get on to any mathematics. The first is that I still think it is important to have as complete a record of our thought processes as is reasonable. So I typed mine into a file as I was having them, and the file is available here to anyone who might be interested. The rest of this post will be a sort of digest of the contents of that file. The second remark is that I am writing this as a post rather than a comment because it feels to me as though it is the beginning of a strand of discussion rather than the continuation of one, though it grows out of some of the comments made on the last post. Note that since we are operating on the Polymath blog, anybody else is free to write a post too (if you are likely to be one of the main contributors, haven’t got moderator status and want it, get in touch and I can organize it).

The starting point for this line of thought is that the main difficulty we face seems to be that Bourgain’s Bohr-sets approach to Roth is in a sense the obvious translation of Meshulam’s argument, but because we have to make a width sacrifice at each iteration it gives a $(\log N)^{-1/2}$ type bound rather than a $(\log N)^{-1}$ type bound. Sanders’s argument gives a $(\log N)^{-1}$ type bound, but if we use that then it is no longer clear how to import the new ideas of Bateman and Katz. Therefore, peculiar as it might seem to jettison one of the two papers that made this project seem like a good one in the first place, it is surely worth thinking about whether the width sacrifice that Bourgain makes (and that is also made in subsequent refinements of Bourgain’s method, due to Bourgain and Sanders) is fundamentally necessary or merely hard to avoid. (more…)

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