For some reason, I just like watching Lloyd's algorithm (https://en.wikipedia.org/wiki/Lloyd%27s_algorithm) in animated form.

My latest puzzle (a belated Christmas present to myself) is the Hanayama Twist. I'm very pleased with it: an elegant symmetric two-piece design, solid feel in the hand, and a solution that is surprisingly complicated but not tediously long. My only complaint is that the solution is very linear, with almost no ways to go wrong if you keep moving on from things you've already done. Anyway here's a map I drew to help me, also used as an example of an implicit graph in my graph algorithms lectures.

Think of a number, and keep a running total starting at 0. Each turn, add your number on to the total. Then, if the old total was a multiple of your number, add one to your number. Otherwise, subtract 1.

The game ends when your number is 1.

Which starting numbers eventually get to 1? In the video, it looks like starting at 4 doesn't, but starting at 2 does.

My new sequence, https://oeis.org/A338807, lists the numbers that eventually reach 1. I'd love to know if there's a pattern!

Complete classification of tetrahedra whose angles are all rational multoples of \(\pi\): https://threadreaderapp.com/thread/1333670741590503425.html, via https://aperiodical.com/2020/12/aperiodical-news-roundup-november-2020/

The original paper is "Space vectors forming rational angles", https://arxiv.org/abs/2011.14232, by Kiran S. Kedlaya, Alexander Kolpakov, Bjorn Poonen, and Michael Rubinstein

I've got a new sequence in the OEIS. It's to do with a really simple number game I made up. Here's a video explaining the rules

https://www.youtube.com/watch?v=ZtGcd0xxqac

Full video at https://youtu.be/ljyJFsaAE84.

The light herder: http://lightherder.blogspot.com/

Dave Blair makes dynamic fractals from old-school video feedback, via https://boingboing.net/2020/10/18/amazing-in-camera-patterns-with-a-video-feedback-kinetic-sculpture.html

Mechanical Computing Systems Using Only Links and Rotary Joints

Article by Ralph C. Merkle and Robert A. Freitas Jr. and Tad Hogg and Thomas E. Moore and Matthew S. Moses and James Ryley

In collections: Basically computer science, Things to make and do, Unusual computers

A new paradigm for mechanical computing is demonstrated that requires only two basic parts, links and rotary joints. These basic parts...

URL: http://arxiv.org/abs/1801.03534v1

PDF: http://arxiv.org/pdf/1801.03534v1

Entry: https://read.somethingorotherwhatever.com/entry/MechanicalComputingSystemsUsingOnlyLinksandRotaryJoints

Mathematician working mostly in three-dimensional geometry and topology, and mathematical artist working mostly in 3D printing and virtual reality.

Joined Apr 2017