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## Runge's Phenomenon

This is a post I wrote for round 2 of The Aperiodical's Big Internet Math-Off 2018. As I went out in round 1 of the Big Math-Off, you got to read about the real projective plane instead of this.

Polynomials are very nice functions: they're easy to integrate and differentiate, it's quick to calculate their value at points, and they're generally friendly to deal with. Because of this, it can often be useful to find a polynomial that closely approximates a more complicated function.

Imagine a function defined for \(x\) between -1 and 1. Pick \(n-1\) points that lie on the function. There is a unique degree \(n\) polynomial (a polynomial whose highest power of \(x\) is \(x^n\)) that passes through these points. This polynomial is called an

*interpolating polynomial*, and it sounds like it ought to be a pretty good approximation of the function.So let's try taking points on a function at equally spaces values of \(x\), and try to approximate the function:

$$f(x)=\frac1{1+25x^2}$$
I'm sure you'll agree that these approximations are pretty terrible, and they get worse as more points are added. The high error towards 1 and -1 is called Runge's phenomenon, and was discovered in 1901 by Carl David Tolmé Runge.

All hope of finding a good polynomial approximation is not lost, however: by choosing the points more carefully, it's possible to avoid Runge's phenomenon. Chebyshev points (named after Pafnuty Chebyshev) are defined by taking the \(x\) co-ordinate of equally spaced points on a circle.

The following GIF shows interpolating polynomials of the same function as before using Chebyshev points.

Nice, we've found a polynomial that closely approximates the function... But I guess you're now wondering how well the Chebyshev interpolation will approximate other functions. To find out, let's try it out on the votes over time of my first round Big Internet Math-Off match.

The graphs below show the results of the match over time interpolated using 16 uniform points (left) and 16 Chebyshev points (right). You can see that the uniform interpolation is all over the place, but the Chebyshev interpolation is very close the the actual results.

But maybe you still want to see how good Chebyshev interpolation is for a function of your choice... To help you find out, I've written @RungeBot, a Twitter bot that can compare interpolations with equispaced and Chebyshev points. Just tweet it a function, and it'll show you how bad Runge's phenomenon is for that function, and how much better Chebysheb points are.

For example, if you were to tweet @RungeBot f(x)=abs(x), then RungeBot would reply: Here's your function interpolated using 17 equally spaced points (blue) and 17 Chebyshev points (red). For your function, Runge's phenomenon is terrible.

A list of constants and functions that RungeBot understands can be found here.

### Similar posts

Mathsteroids | realhats | Harriss and other spirals | Christmas (2018) is over |

### Comments

Comments in green were written by me. Comments in blue were not written by me.

**2018-07-06**

## Mathsteroids

This is a post I wrote for round 1 of The Aperiodical's Big Internet Math-Off 2018, where Mathsteroids lost to MENACE.

A map projection is a way of representing the surface of a sphere, such as the Earth, on a flat surface. There is no way to represent all the features of a sphere on a flat surface, so if you want a map that shows a certain feature of the world, then you map will have to lose some other feature of the world in return.

To show you what different map projections do to a sphere, I have created a version of the game

*Asteroids*on a sphere. I call it*Mathsteroids*. You can play it here, or follow the links below to play on specific projections.### Mercator projection

The most commonly used map projection is the Mercator projection, invented by Gerardus Mercator in 1569. The Mercator projection preserves angles: if two straight lines meet at an angle \(\theta\) on a sphere, then they will also meet at an angle \(\theta\) on the map. Keeping the angles the same, however, will cause the straight lines to no longer appear straight on the map, and the size of the same object in two different place to be very different.

The angle preserving property means that lines on a constant bearing (eg 030° from North) will appear straight on the map. These constant bearing lines are not actually straight lines on the sphere, but when your ship is already being buffeted about by the wind, the waves, and the whims of drunken sailors, a reasonably straight line is the best you can expect.

The picture below shows what three ships travelling in straight lines on the sphere look like on a Mercator projection.

To fully experience the Mercator projection, you can play

*Mathsteroids*in Mercator projection mode here. Try flying North to see your spaceship become huge and distorted.### Gall–Peters projection

The Gall–Peters projection (named after James Gall and Arno Peters) is an area-preserving projection: objects on the map will have the same area as objects on the sphere, although the shape of the object on the projection can be very different to its shape on the sphere.

The picture below shows what three spaceships travelling in straight lines on a sphere look like on the Gall–Peters projection.

You can play

*Mathsteroids*in Gall–Peters projection mode here. I find this one much harder to play than the Mercator projection, as the direction you're travelling changes in a strange way as you move.### Azimuthal projection

An azimuthal projection makes a map on which the directions from the centre point to other points on the map are the same as the directions on the sphere. A map like this could be useful if, for example, you're a radio operator and need to quickly see which direction you should point your aerial to communicate with other points on the map.

The azimuthal projection I've used in

*Mathsteroids*also preserves distances: the distance from the centre to the another points on the map is proportional to the actual distance on the sphere. This projection is used as the emblem of the UN.The picture below shows three straight lines on this projection. You can play

*Mathsteroids*in azimuthal mode here.A retroazimuthal projection makes a map on which the directions to the centre point from other points on the map are the same as the directions on the sphere. If you're thinking that this is the same as the azimuthal projection, then you're too used to doing geometry on flat surfaces: on a sphere, the sum of the angles in a triangle depends on the size of the triangle, so the directions from A to B and from B to A aren't as closely related as you would expect.

The Craig retroazimuthal projection was invented by James Ireland Craig in 1909. He used Mecca as his centre point to make a map that shows muslims across the world which direction they should face to pray.

The picture below shows what three spaceships travelling in a straight lines on a sphere looks like on this projection.

You can play

*Mathsteroids*in Craig retroazimuthal mode here to explore the projection yourself. This is perhaps the hardest of all to play, as (a) two different parts of the sphere overlap on the map, and (b) the map is actually infinitely tall, so quite a bit of it is off the edge of the visible game area.### Stereographic projection

The final projection I'd like to show you is the stereographic projection.

Imagine that a sphere is sitting on a 2D plane. Take a point on the sphere. Imagine a straight line through this point and the point at the top of the sphere. The point where this line meets the 2D plane is stereographic projection of the point on the sphere.

This projection (backwards) can be used to represent the every complex number as a point on a sphere: this is called the Riemann sphere.

To make

*Mathseteroids*playable after this projection, I split the sphere into 2 hemisphere and projected each seperately to give two circles. You can play*Mathsteroids*in stereographic projection mode here. Three spaceships travelling in straight lines on this projection are shown below.... and if you still don't like map projections, you can still enjoy playing ~~head over to The Aperiodical and vote~~ (voting now closed).

*Mathsteroids*on an old fashioned torus. Or on a Klein bottle or the real projective plane. Don't forget to take a short break from playing to### Similar posts

Video game surfaces | Runge's Phenomenon | Optimal Pac-Man | Harriss and other spirals |

### Comments

Comments in green were written by me. Comments in blue were not written by me.

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