## Archive for December, 2009

### The Steinhaus Conjecture

December 23, 2009

Or, perhaps more appropriately, “A” Steinhaus conjecture, he/she (I’m guessing Hugo, so he. Perhaps I’ll look into it) seems to have made a couple. This conjecture (theorem) also goes by the name “The Three Gap Theorem”, or “The Three Distance Theorem”. Which is all a little annoying, I think. It makes looking for references 3 times as hard, I reckon. But it’s a pretty cool result, and I’m glad Dave Richeson brought it to my attention via his blog post on Three cool facts about rotations of the circle.

To write down the theorem, I’ll first introduce the notation $\{x\}$ for the “decimal part” of a real number, defined as $x-\lfloor x\rfloor$, $\lfloor x\rfloor$ being the largest integer no bigger than $x$. Since I’ll be thinking about positive $x$, it is the value of $x$ if you ignore digits to the left of the decimal point. This seems to be fairly common notation. Anyway…

The theorem goes something like this:

Theorem: Suppose that $0<\alpha<1$ is irrational. Let $N$ be a positive integer bigger than 1, and consider the $N$ points $\{m\alpha\}$ for $0\leq m . These points partition the interval $[0,1]$ into $N$ subintervals. If the distances of these subintervals are calculated, there will be either 2 or 3 distinct distances.

The circle comes in by thinking of the interval $[0,1]$ as a circle with circumference 1. To help visualize it, Dr. Richeson made a pretty sweet GeoGebra applet.

I think this is a pretty initially surprising theorem. My initial shock has worn off just slightly, now that I’ve played with pictures and dug through a proof, but it’s still a wonderful result. I mean, irrational values are supposed to do weird things, right? Their multiples should bounce all over the place in the unit interval. And yet, they partition the circle into just 2 or 3 differently-sized gaps? Crazy talk. Also, the theorem as stated above isn’t as strong as it could be… you can say a bit more about the distances. I think I’ll talk more about it in another post.

I started reading about this theorem, after Dr. Richeson’s post, in the paper by Tony van Ravenstein. As I was reading the proof I got hung up on some details, and found that consulting the paper by Micaela Mayero got me over those difficulties. The paper by Mayero is essentially a formal proof written for the Coq formal proof system, so it sort of makes sense that details will be pretty fully explained in it. Either way though, it’s really not a long or particularly difficult proof (you mostly play with some inequalities).

I may return, in a future post, to talking about the proof, and I’ll certainly come back and tell you as I read more about further consequences and generalizations, and whatever else I find in some other papers I’m planning on looking at. But for now, let me mention a result in van Ravenstein’s paper. He proves that in going from the picture with $N$ points to the picture of $N+1$ points, the $N+1$-th point will break up the oldest of the intervals with the largest length. The “age” of an interval is pretty intuitive. If a particular interval, say between multiples $\{p\alpha\}$ and $\{q\alpha\}$ comes in when there are $N_0$ points, and those two points are still neighbors when there are $N_1$ points, then the age of that interval, at stage $N_1$, is $N_1-N_0$ (plus 1, if you want, it doesn’t matter).

To help picture what’s going on, I made an interactive Sage notebook. If you have an account on sagenb.org, or have Sage installed on your own computer and want to just copy my code over, you can look at my code and play with the notebook. I had hoped that publishing my notebook there would let you play with the interactive bits without you needing an account, but no dice. Sorry.

To give some sense of my notebook, and the theorem, I’ve got some pictures for you.

First, let’s take $\alpha=0.3826$ or so (basically 1 minus the golden ratio, nice and irrational). I’ve set up my notebook so that points travel from 0, at the top of the circle, clockwise, because that’s how it was done in the papers I was reading, and I thought it’d be less confusing. So here’s the starting picture, when there’s just the points 0 and $\alpha$:

Along the outside of the circle, at each dot, I list which multiple it is. The “newest” dot is magenta, instead of red (probably not great color choices… mess with my code and make your own :)). In the center of the circle I list the lengths of the intervals, in decreasing order. Along each interval, I also write the age of that interval, and color-code the text to the list of distances. I’ve decided to always have the largest length be red-orangeish, the smallest length blue-ish, and the middle length (if there is one) green-ish.

In the picture above, the interval on the left is clearly the oldest of the longest length intervals, so the theorem is that when we add another point, this interval will get broken up by that point. Which is clearly true in this case.

Here’s another picture, using the same $\alpha$, but slightly more points, showing that three gaps occur:

And, finally, 20 points:

Here’s a picture using a starting $\alpha$ a little bigger than 0.6, showing 20 points:

I like how the points seem to develop in clusters (also evidenced by Dr. Richeson’s app).

I guess that’s probably enough for now. Like I said, I’m hoping to have plenty more to say about things related to all of this soon…

Postscript: I want to make a few shout-outs. I thought putting them at the end of this post might interrupt any sort of flow of the article (if there is any) a little less.

mixedmath pointed out in the comments that public sagenb notebooks are currently (20130623) down. The code looks terrible in the comments, so I figured I’d just add it here:

defalpha = 0.38197 # golden ratio, ish
maxN = 20 # maximum number of points to put in the circle
tolerance = 10^(-7) # to decide when two floats are equal

# some colors, lower index corresponds to bigger distance
segcolor = [(0.86,0.28,0.06),(0.52,0.80,0.06),(0.19,0.11,0.60)]

# the unit circle
basepic = circle((0,0),1,rgbcolor=(0,0,0))

# floating part of a number
flpart = lambda v: v-int(v)

# a point v units along the circumpherence (of length 1 unit) at radius R
coords = lambda v,R: (R*sin(2*pi*v), R*cos(2*pi*v))

# draw dots on the circle, distinguish the "newest" by color
olddot = lambda v:circle(coords(v,1),0.02,rgbcolor=(1,0,0),fill=True)
newdot = lambda v:circle(coords(v,1),0.02,rgbcolor=(1,0,1),fill=True)

# storage
picturestore = {}

""" Make the picture for alphaval and all (up to maxN) numbers of points """
picture = [basepic for m in range(0,maxN+1)] # to go into storage
multiple = [flpart(m*alphaval) for m in range(0,maxN+1)] # the points

# we care most about which distances are longest/shortest, and how
# long each interval has been a certain distances
# we'll build up these next few arrays as we increment the number of points
# disttosucc[m] = actual distance to next point
# agethisdist[m] = how long the interval after the point has been this distance
# distsize[m] = 0,1,2 if the interval after point m is a big,med,or small interval
disttosucc = [1] + [-1 for m in range(0,maxN)]
agethisdist = [1] + [-1 for m in range(0,maxN)]
distsize = [0] + [-1 for m in range(0,maxN)]

# now, build up to having all of the points
# currently, we suppose we only know the 0 point
for N in xrange(2,maxN+1):

newpt = multiple[N-1]

# the new point breaks the oldest interval among those with biggest length
# so find that interval
oldestbigdist = distsize.index(0)
for idx in xrange(oldestbigdist + 1, N-1):
if distsize[idx] == 0 and agethisdist[idx] > agethisdist[oldestbigdist]:
oldestbigdist = idx
# newpt is the successor of oldestbigdist

# update the only distances that change when adding this point
splitdist = disttosucc[oldestbigdist]
disttosucc[oldestbigdist] = newpt - multiple[oldestbigdist]
disttosucc[N-1] = splitdist - disttosucc[oldestbigdist]

# reset the age counts for these two new distances
agethisdist[oldestbigdist] = 1
agethisdist[N-1] = 1

# now we recompute which distances are biggest/smallest

# first, what are the 2 or 3 distances?
distances = [disttosucc[oldestbigdist], 0, disttosucc[N-1]]
if disttosucc[oldestbigdist] < disttosucc[N-1]:
distances[0] = disttosucc[N-1]
distances[2] = disttosucc[oldestbigdist]
for idx in xrange(0,N):
# we're using the fact that there are only 3 distances,
# and that we already know two of them
if disttosucc[idx] - distances[0] > tolerance:
distances[1] = distances[0]
distances[0] = disttosucc[idx]
elif distances[0] - disttosucc[idx] > tolerance:
if distances[2] - disttosucc[idx] > tolerance:
distances[1] = distances[2]
distances[2] = disttosucc[idx]
elif disttosucc[idx] - distances[2] > tolerance:
distances[1] = disttosucc[idx]
# while we're at it, update age of un-changed intervals
if not idx == oldestbigdist and not idx == N-1:
agethisdist[idx] += 1

# now that we know the 2-3 distances, we can tell which points have which dist.
for idx in xrange(0,N):
smidx = 0
while abs(distances[smidx]-disttosucc[idx]) > tolerance:
smidx += 1
distsize[idx] = smidx

# finally, build the picture
dots = [olddot(multiple[m]) for m in xrange(0,N-1)] + [newdot(multiple[N-1])]
labels = [text(str(m),coords(multiple[m],1.1),rgbcolor=(0,0,1))
for m in xrange(0,N)]
agelabels = [text(str(agethisdist[m]),
coords(multiple[m]+.5*disttosucc[m],.9),
rgbcolor=segcolor[distsize[m]])
for m in xrange(0,N)]
distancelegend = text(str(distances[0]),(0,.1),rgbcolor=segcolor[0])
distancelegend += text(str(distances[2]),(0,-.1),rgbcolor=segcolor[2])
if distances[1]:
distancelegend += text(str(distances[1]), (0,0), rgbcolor=segcolor[1])
picture[N] += sum(dots)+sum(labels)+sum(agelabels)+distancelegend

# outside the loop, all pictures have been computed, just store them
picturestore[alphaval] = picture

# set up the interactive bits
@interact
def _( alpha=slider(0.0001,0.9999,0.0001,default=defalpha,label='Distance'),
N=slider(2,maxN,1,default=2,label='Number of Points') ):
if alpha not in picturestore:
show(picturestore[alpha][N], axes=False, aspect_ratio = 1)

### Carnival of Mathematics #60

December 4, 2009

Welcome to the Carnival of Mathematics! Finding that the 60th is apparently the “diamond anniversary,” I was reminded of the symmetry in the Buckyball $C_{60}$, which has the shape of a truncated icosahedron. You can make pretty nice ones using modular origami:

Before getting to this month’s links, allow me a diversion to talk about some geometry I learned a little of this month.

There are 6!=720 ways to order the letters A, B, E, I, L, and S. If we declare that two orderings are the same if one is obtained from the other by cyclic permutation (for example, ABEILS and ILSABE are the same), there are 6!/6=5!=120 combinations. If we also declare that a word and it’s reverse are the same (ABEILS = SLIEBA), we have arrived at 6!/(6*2)=60 combinations.

Pick any 6 distinct points on a circle (or any conic section). Choose any of the points as a starting point, and draw a line to any of the other points. Then draw a line to one of the remaining 4 points. Continue until all of the points have been hit, and then draw a line back to your starting point. How many different pictures can you make in this process? 60, again, because you could label the points A, B, E, I, L, S, and then pictures correspond to words from the previous calculation.

Each picture you draw is a figure with six edges. These six edges can be put into three set of pairs, where two edges are paired if they are “opposite.” In the process of drawing the lines, above, the line opposite the very first line is the fourth line you draw. Similarly, the second and fifth form a pair, and then the third and sixth.

Now, if you extend all of the lines, each pair of opposite edges will determine a point of intersection (or infinity… maybe try another setup for your original points :)). So each picture you draw determines 3 points in the plane (or infinity). When he was only 16, Pascal showed that these three points are always colinear.

So, given 6 points on a conic, the process outlined above determines 60 lines, called Pascal Lines. Mathworld has more on Pascal Lines, for the inquisitive, so it’s probably about time to direct you over there and get on to this month’s blog posts!

In honor of 60 being both a colossally abundant number and a superior highly composite number, I thought it fitting to include as many links as divisors of 60. I ended up with slightly more links than that, so here are $\phi(60)=12$ (more on $\phi$) groups of links from the previous month:

1) At the beginning of the month, Charles Siegel, at Rigorous Trivialities decided to parallel the National Novel Writing Month (NaNoWriMo) by introducing Math Blog Writing Month, MaBloWriMo. After putting it to a vote, he wrote a series on intersection theory. Also taking up MaBloWriMo were Akhil Mathew at Delta Epsilons, Qiaochu Yuan at Annoying Precision, Harrison Brown at Portrait of the Mathematician and, well, yours truly. I found it to be a great experience, and hope next year brings many more authors. If you like your daily math in bite-size fashion, and not just in MaBloWriMo, you might check out probfact on twitter for daily probability facts.

2) At approximately halfway through the month, Wednesday the 18th was determined to be the 150th birthday of the Riemann Hypothesis. Plus Magazine and Math In The News both had articles.

3) Riemann’s zeta function, the lead character in his hypothesis, is connected to primes by Euler’s product formula. If you are interested in the distribution of the primes, Matt Springer at Built on Facts has a post about the function Li(x), as part of his running Sunday Function series. If natural number primes aren’t exciting enough for you, Rich Beveridge at Where The Arts Meet The Sciences has a post for you on Gaussian Primes.

4) It would hardly be a month of math posts without some puzzles:

If you prefer unsolved puzzles, Bill the Lizard has recently written posts about the Collatz Conjecture and the Perfect Cuboid Problem. Alternatively, for some behind-the-scenes on the notoriously difficult Putnam exam (and yet more puzzles), head over to Izabella’s post at The Accidental Mathematician.

5) It’ll take a while to get to the 3435th Carnival of Math, so I think I’m not stepping on too many toes if I point you at Mike Croucher’s quick post at Walking Randomly and Dan MacKinnon’s slightly longer post at mathrecreation that talk about what makes 3435 interesting.

6) Brian, at bit-player, finds some interesting math in a collection of staples, as described in The birth of the giant component.

10) Fëanor at JOST A MON presents Accumulated Causes and Unknowable Effects, related to Pascal’s Wager.

12) This month also saw some nice calculus posts. Daniel Colquitt at General Musings describes the fascinating trumpet of Torricelli. Kalid at BetterExplained asked Why Do We Need Limits and Infinitesimals? and had A Friendly Chat About Whether 0.999… = 1.

15) Kareem at The Twofold Gaze points out that asking for a Best Proximate Value has two reasonable answers.

20) Plus Magazine had an article entitled Pandora’s 3D Box, talking about a recently discovered fractal inspired by the Mandelbrot set.

30) Dave Richeson at Division By Zero reports on a case of mistaken identity in Legendre Who?

60) Finally, Samuel at ACME Science discusses the fractured state of the current mathematics community, noting that Mathematics Really is Discrete. This post was closely followed by Abstruse Goose’s Landscape.

That’s it for now. Look for the next Carnival, Math Teachers at Play, in two weeks!