Some topological fun

A friend sent me this tweet, start of a thread of some mathematically neat parks.

This jungle gym has the shape of one of the classic three-dimensional representations of the Klein bottle. It’s one of pop mathematics’s favorite shapes, up there with the Möbius strip, another all-time favorite.

Both the Klein bottle and the Möbius strip have many possible appearances, for about the same reason there are many kinds of trapezoids or octagons or whatnot. Möbius strips are easy enough to make in real life. Klein bottles, not so; the shape needs four dimensions of space and we just don’t have them. We’ll represent it with a shape that loops back through itself, but a real Klein bottle wouldn’t do that, for the same reason a wireframe cube’s edges don’t intersect the way the lines of its photograph do.

It makes a good wireframe shape, though. I’m surprised not to see more playground equipment using it.

The Summer 2017 Mathematics A To Z: Klein Bottle

Gaurish, of the For The Love Of Mathematics blog, takes me back into topology today. And it’s a challenging one, because what can I say about a shape this involved when I’m too lazy to draw pictures or include photographs most of the time?

In 1958 Clifton Fadiman, an open public intellectual and panelist on many fine old-time radio and early TV quiz shows, edited the book Fantasia Mathematica. It’s a pleasant read and you likely can find a copy in a library or university library nearby. It’s a collection of mathematically-themed stuff. Mostly short stories, a few poems, some essays, even that bit where Socrates works through a proof. And some of it is science fiction, this from an era when science fiction was really disreputable.

If there’s a theme to the science fiction stories included it is: Möbius Strips, huh? There are so many stories in the book that amount to, “what is this crazy bizarre freaky weird ribbon-like structure that only has the one side? Huh?” As I remember even one of the non-science-fiction stories is a Möbius Strip story.

I don’t want to sound hard on the writers, nor on Fadiman for collecting what he has. A story has to be about people doing something, even if it’s merely exploring some weird phenomenon. You can imagine people dealing with weird shapes. It’s hard to imagine what story you could tell about an odd perfect number. (Well, that isn’t “here’s how we discovered the odd perfect number”, which amounts to a lot of thinking and false starts. Or that doesn’t make the odd perfect number a MacGuffin, the role equally well served by letters of transit or a heap of gold or whatever.) Many of the stories that aren’t about the Möbius Strip are about four- and higher-dimensional shapes that people get caught in or pass through. One of the hyperdimensional stories, A J Deutsch’s “A Subway Named Möbius”, even pulls in the Möbius Strip. The name doesn’t fit, but it is catchy, and is one of the two best tall tales about the Boston subway system.

Besides, it’s easy to see why the Möbius Strip is interesting. It’s a ribbon where both sides are the same side. What’s not neat about that? It forces us to realize that while we know what “sides” are, there’s stuff about them that isn’t obvious. That defies intuition. It’s so easy to make that it holds another mystery. How is this not a figure known to the ancients and used as a symbol of paradox for millennia? I have no idea; it’s hard to guess why something was not noticed when it could easily have been It dates to 1858, when August Ferdinand Möbius and Johann Bendict Listing independently published on it.

The Klein Bottle is newer by a generation. Felix Klein, who used group theory to enlighten geometry and vice-versa, described the surface in 1882. It has much in common with the Möbius Strip. It’s a thing that looks like a solid. But it’s impossible to declare one side to be outside and the other in, at least not in any logically coherent way. Take one and dab a spot with a magic marker. You could trace, with the marker, a continuous curve that gets around to the same spot on the “other” “side” of the thing. You see why I have to put quotes around “other” and “side”. I believe you know what I mean when I say this. But taken literally, it’s nonsense.

The Klein Bottle’s a two-dimensional surface. By that I mean that could cover it with what look like lines of longitude and latitude. Those coordinates would tell you, without confusion, where a point on the surface is. But it’s embedded in a four-dimensional space. (Or a higher-dimensional space, but everything past the fourth dimension is extravagance.) We have never seen a Klein Bottle in its whole. I suppose there are skilled people who can imagine it faithfully, but how would anyone else ever know?

Big deal. We’ve never seen a tesseract either, but we know the shadow it casts in three-dimensional space. So it is with the Klein Bottle. Visit any university mathematics department. If they haven’t got a glass replica of one in the dusty cabinets welcoming guests to the department, never fear. At least one of the professors has one on an office shelf, probably beside some exams from eight years ago. They make nice-looking jars. Klein Bottles don’t have to. There are different shapes their projection into three dimensions can take. But the only really different one is this sort of figure-eight helical shape that looks like a roller coaster gone vicious. (There’s also a mirror image of this, the helix winding the opposite way.) These representations have the surface cross through itself. In four dimensions, it does no such thing, any more than the edges of a cube cross one another. It’s just the lines in a picture on a piece of paper that cross.

The Möbius Strip is good practice for learning about the Klein Bottle. We can imagine creating a Bottle by the correct stitching-together of two strips. Or, if you feel destructive, we can start with a Bottle and slice it, producing a pair of Möbius Strips. Both are non-orientable. We can’t make a division between one side and another that reflects any particular feature of the shape. One of the helix-like representations of the Klein Bottle also looks like a pool toy-ring version of the Möbius Strip.

And strange things happen on these surfaces. You might remember the four-color map theorem. Four colors are enough to color any two-dimensional map without adjacent territories having to share a color. (This isn’t actually so, as the territories have to be contiguous, with no enclaves of one territory inside another. Never mind.) This is so for territories on the sphere. It’s hard to prove (although the five-color theorem is easy.) Not so for the Möbius Strip: territories on it might need as many as six colors. And likewise for the Klein Bottle. That’s a particularly neat result, as the Heawood Conjecture tells us the Klein Bottle might need seven. The Heawood Conjecture is otherwise dead-on in telling us how many colors different kinds of surfaces need for their map-colorings. The Klein Bottle is a strange surface. And yes, it was easier to prove the six-color theorem on the Klein Bottle than it was to prove the four-color theorem on the plane or sphere.

(Though it’s got the tentative-sounding name of conjecture, the Heawood Conjecture is proven. Heawood put it out as a conjecture in 1890. It took to 1968 for the whole thing to be finally proved. I imagine all those decades of being thought but not proven true gave it a reputation. It’s not wrong for Klein Bottles. If six colors are enough for these maps, then so are seven colors. It’s just that Klein Bottles are the lone case where the bound is tighter than Heawood suggests.)

All that said, do we care? Do Klein Bottles represent something of particular mathematical interest? Or are they imagination-capturing things we don’t really use? I confess I’m not enough of a topologist to say how useful they are. They are easily-understood examples of algebraic or geometric constructs. These are things with names like “quotient spaces” and “deck transformations” and “fiber bundles”. The thought of the essay I would need to write to say what a fiber bundle is makes me appreciate having good examples of the thing around. So if nothing else they are educationally useful.

And perhaps they turn up more than I realize. The geometry of Möbius Strips turns up in many surprising places: music theory and organic chemistry, superconductivity and roller coasters. It would seem out of place if the kinds of connections which make a Klein Bottle don’t turn up in our twisty world.

In Which I Offer Excuses Instead Of Mathematics

I’d been hoping to get back into longer-form essays. And then the calculations I meant to do on one problem turned out more complicated than I’d wanted. And they’re hard to square with the approach I used in some earlier work. Not that the results I was looking at were wrong, mind, just that an approach I’d used as “convenient for this sort of problem” turned inconvenient here.

So while I have the whole piece back in the shop for re-thinking, which is harder than even thinking, let me give you some other stuff to read. Or look at. One is from regular Singaporean correspondent MathTuition88. If you know anything about topology it’s because you’ve heard about Möbius strips. Surfaces with a single side are neat, and form the base of 95 percent of all science fiction stories in which the mathematics is the fantastic element. Klein bottles are often mentioned as a four-dimensional analogue to the Möbius strip, a solid object with no distinguishable interior or exterior. And a Klein bottle can be divided into two Möbius strips. MathTuition88 showcases a picture about how to turn two strips into a bottle. Or at least the best approximation of a bottle we can do; the actual Klein bottle is a four-dimensional structure and we can just make a three-dimensional imitation of the thing.

For something a bit more vector-analytic Joe Heafner’s Tensor Time has an essay about vectors. It’s about Heafner’s dislike for the way some vector problems are presented. Some common and easy ways to solve vector equations lead to spurious solutions that have to be weeded out by ad hoc reasoning; can’t we do better? Heafner argues that we can and should. The suggested alternative looks a little stuffy, but as often happens, spending more time on the setup means one spends less time confused later on. Worth pondering.

And this is a late addition, but I couldn’t resist.

Now I have a new favorite first chapter for a calculus text.

Reading the Comics, January 15, 2015: Electric Brains and Klein Bottles Edition

I admit I don’t always find a theme running through Comic Strip Master Command’s latest set of mathematically-themed comics. The edition names are mostly so that I can tell them apart when I see a couple listed in the Popular Posts roundup anyway.

Jimmy Hatlo’s Little Iodine is a vintage comic strip from the 1950s. It strikes me as an unlicensed adaptation of Baby Schnooks, but that’s not something for me to worry about. The particular strip, originally from the 7th of November, 1954 (and just run the 12th of January this year) interests me for its ancient views of computers. It’s from the days they were called “electric brains”. I’m also impressed that the machine on display early on is able to work out the “square root of 7921 x2 y2”. The square root of 7921 is no great feat. Being able to work with the symbols of x and y without knowing what they stand for, though, does impress me. I’m not sure there were computers which could handle that sort of symbolic manipulation in 1954. That sort of ability to work with a quantity by name rather than value is what we would buy Mathematica for, if we could afford it. It’s also at least a bit impressive that someone knows the square of 89 offhand. All told, I think this is my favorite of this essay’s set of strips. But it’s a weak field considering none of them are “students giving a snarky reply to a homework/exam/blackboard question”.

Joe Martin’s Willy and Ethel for the 13th of January is a percentages joke. Some might fault it for talking about people giving 110 percent, but of course, what is “100 percent”? If it’s the standard amount of work being done then it does seem like ten people giving 110 percent gets the job done as quickly as eleven people doing 100 percent. If work worked like that.

Steve Sicula’s Home and Away for the 13th (a rerun from the 8th of October, 2004) gives a wrongheaded application of a decent principle. The principle is that of taking several data points and averaging their value. The problem with data is that it’s often got errors in it. Something weird happened and it doesn’t represent what it’s supposed to. Or it doesn’t represent it well. By averaging several data points together we can minimize the influence of a fluke reading. Or if we’re measuring something that changes in time, we might use a running average of the last several sampled values. In this way a short-term spike or a meaningless flutter will be minimized. We can avoid wasting time reacting to something that doesn’t matter. (The cost of this, though, is that if a trend is developing we will notice it later than we otherwise would.) Still, sometimes a data point is obviously wrong.

Zach Weinersmith’s Saturday Morning Breakfast Cereal wanted my attention, and so on the 13th it did a joke about Zeno’s Paradox. There are actually four classic Zeno’s Paradoxes, although the one riffed on here I think is the most popular. This one — the idea that you can’t finish something (leaving a room is the most common form) because you have to get halfway done, and have to get halfway to being halfway done, and halfway to halfway to halfway to being done — is often resolved by people saying that Zeno just didn’t understand that an infinite series could converge. That is, that you can add together infinitely many numbers and get a finite number. I’m inclined to think Zeno did not, somehow, think it was impossible to leave rooms. What the paradoxes as a whole get to are questions about space and time: they’re either infinitely divisible or they’re not. And either way produces effects that don’t seem to quite match our intuitions.

The next day Saturday Morning Breakfast Cereal does a joke about Klein bottles. These are famous topological constructs. At least they’re famous in the kinds of places people talk about topological constructs. It’s much like the Möbius strip, a ribbon given a twist and joined back to its edge. The Klein bottle similarly you can imagine as a cylinder stretched out into the fourth dimension, given a twist, then joined back to itself. We can’t really do this, what with it being difficult to craft four-dimensional objects. But we can imagine this, and it creates an object that doesn’t have a boundary, and has only one side. There’s not an inside or an outside. There’s no making this in the real world, but we can make nice-looking approximations, usually as bottles.

Ruben Bolling’s Super-Fun-Pak Comix for the 13th of January is an extreme installment of Chaos Butterfly. The trouble with touching Chaos Butterfly to cause disasters is that you don’t know — you can’t know — what would have happened had you not touched the butterfly. You change your luck, but there’s no way to tell whether for the better or worse. One of the commenters at Gocomics.com alludes to this problem.

Jon Rosenberg’s Scenes From A Multiverse for the 13th of January makes quite literal quantum mechanics talk about probability waves and quantum foam and the like. The wave formulation of quantum mechanics, the most popular and accessible one, describes what’s going on in equations that look much like the equations for things diffusing into space. And quantum mechanical problems are often solved by supposing that the probability distribution we’re interested in can be broken up into a series of sinusoidal waves. Representing a complex function as a set of waves is a common trick, not just in quantum mechanics, because it works so well so often. Sinusoidal waves behave in nice, predictable ways for most differential equations. So converting a hard differential equation problem into a long string of relatively easy differential equation problems is usually a good trade.

Tom Thaves’s Frank and Ernest for the 14th of January ties together the baffling worlds of grammar and negative numbers. It puts Frank and Ernest on panel with Euclid, who’s a fair enough choice to represent the foundation of (western) mathematics. He’s famous for the geometry we now call Euclidean. That’s the common everyday kind of blackboards and tabletops and solid cubes and spheres. But among his writings are compilations of arithmetic, as understood at the time. So if we know anyone in Ancient Greece to have credentials to talk about negative numbers it’s him. But the choice of Euclid traps the panel into an anachronism: the Ancient Greeks just didn’t think of negative numbers. They could work through “a lack of things” or “a shortage of something”, but a negative? That’s a later innovation. But it’s hard to think of a good rewriting of the joke. You might have Isaac Newton be consulted, but Newton makes normal people think of gravity and physics, confounding the mathematics joke. There’s a similar problem with Albert Einstein. Leibniz or Gauss should be good, but I suspect they’re not the household names that even Euclid is. And if we have to go “less famous mathematician than Gauss” we’re in real trouble. (No, not Andrew Wiles. Normal people know him as “the guy that proved Fermat’s thing”, and that’s too many words to fit on panel.) Perhaps the joke can’t be made to read cleanly and make good historic sense.