Pinned toot

Hallo, new Mastodon users! Here on mathstodon.xyz we've got a couple of mathematical emoji: and . I'd love to add some more. If you can make a PNG image the same size as those, send it to me and I'll add it. Faces of famous mathematicians are an easy place to start; could we have some shapes, polyhedra, or other mathographics too?

@chalkdustmag @mscroggs @Pecnut thanks for the Christmas card! Hopefully it hadn't been sitting in my pigeonhole for too long

I've just updated mathstodon.xyz to v2.6.5. Basically no fun new features, just some tiny bug fixes from the look of it.

Have just been notified I have 1.5GB of my data allowance left, and it renews tomorrow. I also have 5 hours until my flight...

Rigid Foldability is NP-Hard: arxiv.org/abs/1812.01160

It was previously known that folding a purported origami folding pattern to a flat state is NP-hard, because you can encode logic in the way the paper gets in the way of itself. But this paper proves that it's hard even to tell whether you can make any rigid motion at all starting from completely unfolded paper, well before self-interference kicks in. Instead, the difficulty involves getting sums of angles to come out right.

A 'sum' is a sequence of terms joined by addition.
A 'product' is a sequence of terms joined by multiplication.
Is there a general term for terms joined by a general associative operation, that mathematicians would know? Is it 'sequence'?
In Haskell this would be implemented as a fold, but what do you call the thing it acts on? 'Iterable' and 'Enumerable' are too computer-sciencey.
It's possible this question has no good answer.

I recommend @mscroggs's maths puzzle advent calendar. mscroggs.co.uk

@henryseg have you seen handsfree.js.org/ ? Could be a good way to control things like Hypernom without a VR headset

tooting in Old English is also known as "being þorny on main"

274. THE MOUSE-TRAP PUZZLE. This is a modern version, with a difference, of an old puzzle of the same name. Number twenty-one cards, 1, 2, 3, etc., up to 21, and place them in a circle in the particular order shown in the illustration. These cards represent mice. You start from any card, calling that card
"one," and count, "one, two, three," etc., in a clockwise direction, and when your count agrees with the number on the card, you have made a (1/4)

I'm bringing zenzi back.

Video player uses arrow keys to jump back and forth: 👍
... in increments of one minute: 😭

I'm going to Amsterdam next week, so a quick check of the Zwarte Piet situation...

Dutch people still inexplicably happy to black up: dutchnews.nl/news/2018/12/half

Web people, what registrar do you use for .com domains? I'll accept a slightly higher price for a reasonable management interface.

New blog post: Triply-Hamiltonian edge colorings
11011110.github.io/blog/2018/1

In mathstodon.xyz/@mjd/1010988538, Mark Jason Dominus (@mjd) observed that the regular dodecahedron can have its edges properly colored with three colors so that every two colors form a Hamiltonian cycle. In this post, I survey constructions for more graphs like this, and prove that no planar bipartite graph can have a coloring like this.

Send help

I have set my phone's device name to "Pology". So now whenever I use my bluetooth headset, I hear "Connected to-Pology".

current status: lexicographic parity check matrix hell

Grace Hopper, protect us from bugs

mathstodon.xyz was down overnight because redis decided it couldn't write to disk. Restarting the redis service seems to have fixed it.
We have 21GB free, so it wasn't a lack of space. Who knows! 🤷‍♂️

A Mastodon instance for maths people. The kind of people who make $\pi z^2 \times a$ jokes.
Use $ and $ for inline LaTeX, and $ and $ for display mode.