@andrewt @chalkdustmag @mscroggs @Pecnut no, but the message inside refers to something which took place this year

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

@loke not in my experience

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.

@cg505 wrong order: everyone's mad, so this guy grabbed a giant hammer. It's an important part of our constitution (still gloriously unwritten, so we are Calvinball: The Country)

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...

@ccppurcell and the intro para says 'sequences of elements', so I think I'll go with 'sequences'.

@pkra I didn't know you had maths A level!
Yes, 'doing sums' is often used to mean any calculation.

@amiloradovsky yeah, I'm not interested in those generalisations for the thing I'm working on

@amiloradovsky yeah, I wanted to know: in something like fold f x, what kind of thing is x? The haskell wiki says "a data structure", which is far too broad. I'm thinking of an ordered set, so 'sequence' is the best I can come up with

@amiloradovsky you're naming the process, and I want a name for the terms, considered together

@pkra yeah, that's unfortunate, because it's such an overloaded term. If I say 'products', then a large portion of my readers will assume it doesn't include sums

@loke in any other case, yes, but I'm talking about the form of the thing here, so you don't actually apply the operation

@cpsdqs can't work on an empty stomach

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

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.