Zoppo Trump

In 1969, almost 50 years ago, Zoppo Trump wanted to become the new ruler of the Little People. To avoid the three contests required to challenge the current ruler, he resorted to all sorts of trickery, including kidnapping and attempted murder.
I am making this up? No, this is all written down here, in Tilde Michels’ fabulous book:


This was one of my favorite childhood books. I tells how Jenny and Max discover the King Kalle Wirsch inside a garden gnome, and go on a journey into the realm of the Erdmännchen (Little People), after conveniently being shrunk to proper size by eating raxel root. The meet endearing characters like the ferryman

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and travel inside a fire worm through lava fields

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in order to reach the convention of the Little People just in time to prevent Zoppo Trump to seize power. Here is the bad guy after winning the first contest:

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The story ends dramatically. Maybe you can guess what happens from the last image:


The three middle images are screen grabs from the DVD, made by the famous Augsburger Puppenkiste. As a kid I was glued to the screen whenever this was broadcast. The top image is the front cover of the first edition, and the last image is from within the book, one of the many illustrations by Rüdiger Stoye.

This book has, to my knowledge, not been translated into English. Anyone? Now is the time!


Just Triangles (Polyforms III)

In a former, more optimistic life, I wanted to write a book for elementary school children that would get them excited about math and proofs. This would of course go against the grain. Proofs have essentially been eliminated from all education until the beginning of graduate school. With good & evil reason: Not because they are too difficult or not important enough, but because it could possibly induce the children to come to their own conclusions.

I also was ignorant about who controls public education: Neither the students, nor their parents, nor the teachers, and not even the text book authors. It is solely those people who are making money with it.

Before I get the reputation to be yet another hopeless conspirationist, here is another message in a bottle, in multiple parts. It is once again about polyforms. I need to say what the shapes are that we are allowed to use, and what we want to do with them. In the simplest, we are using four shapes, which are deflated/inflated triangles like so:

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They already have received names, which count the number of edges that have been inflated. We (you) are going to tile shapes like these that have no corners:

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We call these circular regions, because they consist essentially of a few touching circles with a bit of filling to avoid holes or corners. The circular regions above consist of two, three, and seven circles, respectively, and they already have been tiled. We can start asking questions: What shapes without corners can you come up with? Are they all circular regions?

Then there is time for exploration: Find all ways to tile a circle (the circular region with just one circle) with the curved triangles. Find all ways to tile the bone (the circular region with just two circles) using only two different kinds of triangles:

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Now, upon experimenting, the number of curved triangles to be used to tile a circular region is not quite arbitrary.
The first, not so trivial, observation is that for a circular with N circles, we will need 8N-2 triangles. That is because each circle contributes 6 triangles, and for adding a circle we have to use 2 more triangles. This is not quite a proof yet, but at least an argument. There are also interesting problems when the domain is allowed to contain holes…

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Because the different curved triangles contribute a different amount of area each, there is a second formula.
Let’s denote by N(0), N(1), N(2), and N(3) the number of curved triangles of each type (zero, one, two, three) that appear in a tiling of a circular region with N circles. Then N(1)+2N(2)+3N(3) = 12N. This formula counts on the left hand side how many triangle edges are inflated and hence contribute extra area. On the right hand side, we count the same, using say the pattern we see in the tiling of the circular region with 7 circles in the second image above.

The two formulas together allow you to determine how many triangles of each kind you need in an N-circle region, if you are only using two different triangles.

To be continued?

Copycat (Election Games II)

My popular series of election games continues with a paper and pencil game for any number of player. It’s called Copycat. Let’s play the multiplayer version first. Each player grabs a sheet of paper and a pen, draws a rectangular grid of agreed size (I use 4×4 below, 6×6 to 8×8 is better for actual play), and marks an agreed number (I use 1 below, two or three is much better) of intersections with a nice, fat dot.

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One player decides to go first and announces one of the four main compass directions. Now all players have to mark a segment beginning at any one of their dots and heading that way one unit. Above, the first player (left) decided NORTH (where else?), and all players had to follow. A player who can’t follow is out.

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Now it’s the second player’s turn (middle), and she decides EAST. All players have to mark a segment that begins at the endpoint of any one of their paths and moves east one unit, thereby neither retracing steps, nor leaving the grid, nor ending on intersections that have already been visited by any path. The third player (right) has now only two options left (NORTH or SOUTH), and decides NORTH. This eliminates the middle player, who is out of moves.

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Left takes revenge and moves EAST, which is impossible for the right player. This leaves left as the winner. In an (unrealistic) cooperative play, left and right could instead have continued on for eight more moves. The game becomes more interesting when the players begin with more than one dot, because then they can choose which path they extend at each turn.

To make puzzles for single players, start with a board, place a couple of dots, and draw legal paths like so:

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Record the directions along each path as a sequence of letters, namely WNENESSSWWSEE and NESSWSESW in the case above.
Randomly splice the sequences into one, for instance into WNNEENESSSSWSSWWESSEWE. Then draw a new board that just includes the dots, and hand it together with the letter sequence to your best friend. She then needs to trace non-intersecting paths, following the letters as compass directions. Her only choice at each step is which path she wants to extend. This is an excellent example of an easy to make puzzle that is ridiculously hard to solve.

There are many variations: For single players, you can use an eight sided compass die or a spinner to determine the direction at each step.

Several players can also share boards, as long as they can agree on where north is. They would then use pens in different colors and could only extend their own paths, avoiding any crossings of paths.

Just a Circle …

Imagine eight points, nicely spaced and colored, on a circle. Project them stereographically onto a line, keeping their color. In the image below, the projection center is at the top of the circle. You expect to get eight points on the line. One of them is hiding off the screen in the figure below.


Now let’s move the points counterclockwise, with constant speed, around the circle. Their stereographic images will slide along the line. To visualize that motion, we trace the position by using the x-coordiante for the position and the y-coordinate as time. This way we get the colored curves above, each representing two full turns of the point around the circle.

A simple rotation of an 8-gon has become quite tricky. It will get worse. Let’s place two circles (blue and red) onto a sphere, by taking the 45 degree latitudes. When we stereographically project these into the plane, we get two concentric circles. Now rotate the sphere about the y-axis. After 90 degrees, the two circles have become vertical (yellow and green), and their stereographic images are two disjoint circles. How did that happen?

Let’s visualize the process the same way as before, by tracing the position of the circles using the z-coordinate for the angle of rotation.


We obtain two interlocking identical surfaces, both of which have circular horizontal slices. This is reminiscent of Riemann’s minimal surface, but it is not the same surface. Riemann’s surface is several magnitudes more complicated. After all, we are only rotating a sphere.

We can make this a bit more complicated by simultaneously rotating the sphere about the z axis. In other words, we rotate the image circles about the z-axis depending on their height.


Finally, here is the same construction with three circles. This gets quite crowded.



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I am one of those people who are often oblivious of their surroundings, which gives me the advantage to discover things even after years at the same place.

One of these things is Jerald Jacquard’s steel sculpture February, in front of the McCalla School in Bloomington.

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It first caught my attention through the sound it makes: Put one ear next to one of the three “legs” of the sculpture, and gently hit another part. Some ambient musician should explore this.


But the sculpture has more to offer. It is made of 28 blocks (one for each day of February). Each block is either a cube, or a halved cube. For halving a cube Jacquard uses two possibilities, both prisms over isosceles triangles, and both exactly half the volume of the cube. The usage of the (in my re-rendering, red) prisms is strictly limited to the lower part of the sculpture, making it to appear more open at the bottom than at the top.


The other (green) prisms are used to create roof-like slopes. Almost all blocks are placed in a cubical grid, but there is one exception.


The front-bottom cube in the image above is moved to be able to support the two prisms above. Maybe, in leap years, one should add another cube?

Deceiving Simplicity (Annuli VI)


Just three months before his death on July 20, 1866 (150 years ago), Bernhard Riemann handed a few sheets of paper with formulas to Karl Hattendorff, one of his colleagues in Göttingen.
Hattendorff did better than Riemann’s house keeper who discarded the papers and notes she found.

He instead worked out the details, and published this as a posthumous paper of Riemann. It contains his work on minimal surfaces. Riemann was possibly the first person who realized that the Gauss map of a minimal surface is conformal, and that its inverse is well suited to find explicit parametrizations. He used this insight to construct the minimal surface family that bears his name, as well as a few others that were later rediscovered by Hermann Schwarz.


Above is one of Riemann’s minimal surfaces, parametrized by the inverse of the Gauss map. This means in particular that the surface normal along the parameter lines traces out great circles on the sphere. Riemann discovered these surfaces by classifying all minimal surfaces whose intersections with horizontal planes are lines or circles. These are the catenoid, the helicoid, or Riemann’s new 1-parameter family.


The proof utilizes elliptic functions, which is not surprising: Riemann’s minimal surfaces are translation invariant, and their quotient by this translation is a torus, on which the Gauss map is a meromorphic function of degree 2. It is in fact one of the simplest elliptic functions, and one can use it to parametrize Riemann’s surfaces quite elegantly. What is not simple is the proof that these surfaces have indeed circles as horizontal slices. All arguments I know involve some more or less heavy computation. We are clearly lacking some insight here.


The longer one studies these surfaces, the more perplexing they become. There is, for instance, Max Shiffman’s theorem from 1956. It states that if a minimal cylinder has just two horizontal circular slices, all its horizontal slices are circles. The proof is elegant, magical, and still mysterious, just like Riemann’s minimal surfaces.

Inside riemann

Arbeit und Struktur

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As hinted at in a previous post, I have been spending a fair amount of time this summer preparing 3D models for clay printing. I will talk about the models and the results at a later point. Today, we focus (or de-focus?) on watching the process. Printing a model takes time (say two hours for a model 20 cm in width) and requires almost permanent attention.

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So one naturally begins to pay attention to details. The shallow focus of a macro lens not only allows to pinpoint these details, it also blurs everything else into pleasant abstraction.

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Color is almost irrelevant, unless one wants to bring out the gradual change of clay type from layer to layer. Everything is reduced to utter simplicity, to the extent that the all too human question for meaning is becoming meaningless.

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What matters is structure, and the work to be done to maintain it.

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Arbeit und Struktur (Work and Structure) is the title of Wolfgang Herrndorf’s Blog-Diary that he wrote in the last three years of his life.
This diary distills much of what mattered to him while facing death, and the title is a further reduction of this to just two words.