Since 2010, I’ve been maintaining a list of “interesting esoterica” – papers, books, essays and poems that I find interesting entirely on their own merits. It’s mainly bits of esoteric maths – hence the name – but I’ve also included quite a few things just because they have amusing titles. The main idea is that when I’m talking to someone and want to show them a cool thing that I’ve half-remembered, I can look up the exact reference: I’ve shared the paper “Orange peels and Fresnel integrals” more times than I can count (probably the same as the number of times I’ve eaten an orange).

# You're reading: cp’s mathem-o-blog

### Maths Object: Nobbly Wobbly

My maths object this time is one of my dog’s favourite toys: the Nobbly Wobbly.

In the video, I said it was invented by a mathematician, but Dick Esterle’s bio normally goes “artist, architect, inventor”. I’ll leave it up to you to decide if Everyone’s a Mathematician.

It’s a particularly pleasing rubbery ball thing made of six interwoven loops in different colours, invented by Dick Esterle.

On Google+, various people told me the unexpected fact that the outer automorphism group of $S_6$ is hiding inside this dog toy.

I’ve also found this Celebration of Mind livestream starring Dick Esterle from 2013 talking about all sorts of mathematically-shaped toys, including the Nobbly Wobbly.

### Are you sure 51 isn’t prime? – Analysing the results of the “Is this prime?” game

Two months ago, I bought isthisprime.com and not only set up the internet’s fanciest primality-checking service, but also invented a rather addictive game.

It quite quickly went viral, or as relatively viral as a maths game can get, with people tweeting their high scores and posting the link to reddit and Hacker News. I realised fairly soon that I should put in some stats tracking, to see if there were any interesting patterns in the data (and also to inflate my ego as the “games played” counter went up). I missed the first big spike in traffic, but on the 9th of March I wrote a script which saved a record of each game to a database.

The mad rush settled down quite quickly but there were still occasional spikes as different sites or people with lots of twitter followers found the game. Now, after two months, I’ve got data for just under 350,000 games. That’s a decent amount of information!

### Approximate a ratio by folding a piece of paper

*Warning: you could make a very strong argument I’ve thought far too much about something inconsequential. If that makes your stomach turn, look away now.*

This morning in the shower, I had an idle thought about my towel. It was, as always, folded neatly on the toilet seat. A problem that’s been bugging me for a few days is how to pick up the towel by a section of the long edge, so when it unfolds it’s the right way round.

* quiet in the back

The problem is that the short edge and the long edge look the same, and once I’ve folded the towel over a couple of times and had a shower only a madman* would remember which is which. But my towel isn’t square, so it occurred to me that either the longer or the shorter edge, after folding, could be the edge I want. Since I never make a diagonal fold, the long edge is only ever folded on top of the long edge, and likewise for the short edge. I fold the towel until it fits comfortably on top of the toilet seat, and by the time I’ve finished my shower I can’t be relied upon to remember which sequence of folds I did.

Which got me thinking about the ratio between the width and height of my towel: if I know this ratio then, by looking at the towel and counting the number of folds, I can work out which folds I’ve done, and hence which of the sides will unfold to be the long edge.

### Maths Object: Correntator

Here’s another one of my favourite maths objects: the Correntator. It’s a simple mechanical tool to add up amounts of money. I bought it for about a tenner (new money) at a market.

*This video is extremely shonky. Blame my phone, which can’t bring itself to record for more than 250 seconds at a time.*

**More information** about the Correntator.

### I bought isthisprime.com

Around about exactly this time a year ago, I bought the frivolous domain name three.onefouronefivenine.com, to celebrate π Day and to indulge my curiosity about a marvellous algorithm to compute π’s digits.

This year, I’ve been thinking about prime numbers, and my hosting provider has run another sale on domain names. So, I’ve bought isthisprime.com. You can probably guess what I’ve made it do.

### Maths Objects: polyhedra

Time for some more maths objects! This time I wanted to show you the various polyhedra I’ve got around my desk.

- The tetrahedron is made out of a paper plate, following the instructions on the brilliantly kooky wholemovement.
- The sonobe cube is a classic. Mine’s made out of Post-It notes.
- The swirly thing is made out of curler units. Here’s a nice lady explaining how to use them to make a few different polyhedra.
- The classic reference for the Post-It note dodecehadron is James Grime’s video instructions.
- Once you can make a dodecahedron, add some more maths by edge-colouring it. I followed Julia Collins’ 5-colouring. Or if you’re more adventurous and less colourblind, look at George Hart’s colourings page for some really sophisticated patterns.
- I can’t remember how I made the icosahedron. Can anyone remind me?
- Finally, I’ve shown off the enneahedron loads of times. I wrote about its creation a couple of years ago.