## My Little 2021 Mathematics A-to-Z: Torus

Mr Wu, a mathematics tutor in Singapore and author of the blog about that, offered this week’s topic. It’s about one of the iconic mathematics shapes.

# Torus

When one designs a board game, one has to decide what the edge of the board means. Some games make getting to the edge the goal, such as Candy Land or backgammon. Some games set their play so the edge is unreachable, such as Clue or Monopoly. Some make the edge an impassible limit, such as Go or Scrabble or Checkers. And sometimes the edge becomes something different.

Consider a strategy game like Risk or Civilization or their video game descendants like Europa Universalis. One has to be able to go east, or west, without limit. But there’s no making a cylindrical board. Or making a board infinite in extent, side to side. Instead, the game demands we connect borders. Moving east one space from just-at-the-Eastern-edge means we put the piece at just-at-the-Western-edge. As a video game this is seamless. As a tabletop game we just learn to remember those units in Alberta are not so far from Kamchatka as they look. We have the awkward point that the board doesn’t let us go over the poles. It doesn’t hurt game play: no one wants to invade Russia from the north. We can represent a boundless space on our table.

Sometimes we need more. Consider the arcade game Asteroid. The player’s spaceship hopes to survive by blasting into dust asteroids cluttered around them. The game ‘board’ is the arcade screen, a manageable slice of space. Asteroids move in any direction, often drifting off-screen. If they were out of the game, this would make victory so easy as to be unsatisfying. So the game takes a tip from the strategy games, and connects the right edge of the screen to the left. If we ask why an asteroid last seen moving to the right now appears on the left, well, there are answers. One is to say we’re in a very average segment of a huge asteroid field. There’s about as many asteroids that happen to be approaching from off-screen as recede from us. Why our local work destroying asteroids eliminates the off-screen asteroids is a mystery for the ages. Perhaps the rest of the fleet is also asteroid-clearing at about our pace. What matters is we still have to do something with the asteroids.

Almost. We’ve still got asteroids leaking away through the top and bottom. But we can use the same trick the right and left edges do. And now we have some wonderful things. One is a balanced game. Another is the space in which ship and asteroids move. It is no rectangle now, but a torus.

This is a neat space to explore. It’s unbounded, for example, just as the surface of the Earth is. Or (it appears) the actual universe is. Set your course right and your spaceship can go quite a long way without getting back to exactly where it started from, again much like the surface of the Earth or the universe. We can impersonate an unbounded space using a manageably small set of coordinates, a decent-size game board.

That’s a nice trick to have. Many mathematics problems are about how great blocks of things behave. And it’s usually easiest to model these things if there aren’t boundaries. We can, sure, but they’re hard, most of the time. So we analyze great, infinitely-extending stretches of things.

Analysis does great things. But we need sometimes to do simulations, too. Computers are, as ever, great tempting setups to this. Look at a spreadsheet with hundreds of rows and columns of cells. Each can represent a point in space, interacting with whatever’s nearby by whatever our rule is. And this can do very well … except these cells have to represent a finite territory. A million rows can’t span more than one million times the greatest distance between rows. We have to handle that.

There are tricks. One is to model the cells as being at ever-expanding distances, trusting that there are regions too dull to need much attention. Another is to give the boundary some values that, we figure, look as generic as possible. That “past here it carries on like that”. The trick that makes rhetorical sense to mention here is creating a torus, matching left edge to right, top edge to bottom. Front edge to back if it’s a three-dimensional model.

Making a torus works if a particular spot is mostly affected by its local neighborhood. This describes a lot of problems we find interesting. Many of them are in statistical mechanics, where we do a lot of problems about particules in grids that can do one of two things, depending on the locale. But many mechanics problems work like this too. If we’re interested in how a satellite orbits the Earth, we can ignore that Saturn exists, except maybe as something it might photograph.

And just making a grid into a torus doesn’t solve every problem. This is obvious if you imagine making a torus that’s two rows and two columns linked together. There won’t be much interesting behavior there. Even a reasonably large grid offers problems. There might be structures larger than the torus is across or wide, for example, worth study, and those will be missed. That we have a grid means that a shape is easier to represent if it’s horizontal or vertical. In a real continuous space there’s no directions to be partial to.

There are topology differences too. A famous result shows that four colors are enough to color any map on the plane. On the torus we need at least seven. Putting colors on things may seem like a trivial worry. But map colorings represent information about how stuff can be connected. And here’s a huge difference in these connections.

This all is about one aspect of a torus. Likely you came in wondering when I would get to talking about doughnut shapes, and the line about topology may have readied you to hear about coffee cups. The torus, like most any mathematical concept familiar enough ordinary people know the word, connects to many ideas. Some of them have more than one hole. Some have surfaces that intersect themselves. Some extend into four or more dimensions. Some are even constructs that appear in phase space, describing ways that complicated physical systems can behave. These are all reflections of this shape idea that we can learn from thinking about game boards.

## Reading the Comics, April 5, 2016: April 5, 2016 Edition

I’ve mentioned I like to have five or six comic strips for a Reading The Comics entry. On the 5th, it happens, I got a set of five all at once. Perhaps some are marginal for mathematics content but since when does that stop me? Especially when there’s the fun of a single-day Reading The Comics post to consider. So here goes:

Mark Anderson’s Andertoons is a student-resisting-the-problem joke. And it’s about long division. I can’t blame the student for resisting. Long division’s hard to learn. It’s probably the first bit of arithmetic in which you just have to make an educated guess for an answer and face possibly being wrong. And this is a problem that’ll have a remainder in it. I think I remember early on in long division finding a remainder left over feeling like an accusation. Surely if I’d done it right, the divisor would go into the original number a whole number of times, right? No, but you have to warm up to being comfortable with that.

Ted Key’s Hazel feels less charmingly out-of-date when you remember these are reruns. Ted Key — who created Peabody’s Improbable History as well as the sitcom based on this comic panel — retired in 1993. So Hazel’s attempt to create a less abstract version of the mathematics problem for Harold is probably relatively time-appropriate. And recasting a problem as something less abstract is often a good way to find a solution. It’s all right to do side work as a way to get the work you want to do.

John McNamee’s Pie Comic is a joke about the uselessness of mathematics. Tch. I wonder if the problem here isn’t the abstractness of a word like “hypotenuse”. I grant the word doesn’t evoke anything besides “hypotenuse”. But one irony is that hypotenuses are extremely useful things. We can use them to calculate how far away things are, without the trouble of going out to the spot. We can imagine post-apocalyptic warlords wanting to know how far things are, so as to better aim the trebuchets.

Percy Crosby’s Skippy is a rerun from 1928, of course. It’s also only marginally on point here. The mention of arithmetic is irrelevant to the joke. But it’s a fine joke and I wanted people to read it. Longtime readers know I’m a Skippy fan. (Saturday’s strip follows up on this. It’s worth reading too.)

Bill Griffith’s Zippy the Pinhead has picked up some quantum mechanics talk. At least he’s throwing around the sorts of things we see in pop science and, er, pop mathematical talk about the mathematics of cutting-edge physics. I’m not aware of any current models of everything which suppose there to be fourteen, or seventeen, dimensions of space. But high-dimension spaces are common points of speculation. Most of those dimensions appear to be arranged in ways we don’t see in the everyday world, but which leave behind mathematical traces. The crack about God not playing dice with the universe is famously attributed to Albert Einstein. Einstein was not comfortable with the non-deterministic nature of quantum mechanics, that there is this essential randomness to this model of the world.