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The thing with tensors is that they only make sense as a term in the context where they're used (diffgeo), and if you try to explain them in terms of vector analysis, the point gets quickly lost.

Absolute heights of decadence: I wish there was a ligature for \(\omega t\)

Here's an amusing if minor repeated typo in the literature: "appiled superconductivity", (177 hits). I think the source is IEEE, which spells Trans. on Applied Superconductivity correctly on its site but misspells it repeatedly in the doi database. So if you get your citations from doi metadata, you will get this error.

You can see the metadata for a doi by doing curl -LH "Accept: application/x-bibtex" on the url for the doi. Try e.g. 10.1109/TASC.2005.849553

1. Click link
2. It's
3. Close tab

Corker of a line from

“And the story of computation has been about the evolution of this very novel and peculiar form of human expression we call code. I suspect being a programmer in the 21st century must be like what being a royal scribe was like in Ancient Egypt in 3200 BCE. There’s this new modality of communication that most of the population is unaware of, yet it’s existence simultaneously enables commerce, culture and civilization to flourish.”

3-year-old told me that she got a sticker at playgroup for staying quiet while they thought about puppies this morning.

Something's been lost along the way there...

Rookie mistake by this correspondent: mentioned that their problem needs to be resolved by January 2021.

Anything later than three months ago is going to the bottom of the pile

Here's what it means:

I have a process that begins by picking a number N, and totting up a total T that begins at 0.
Repeatedly do this:

* add N to T
* if N divides T, add 1 to N, otherwise subtract 1
* if N is 1, stop

I searched for N where the process stops

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oeiswhack: enter a short, meaningful, sequence of integers into, returning no results.

I think this is probably the best oeiswhack I'm ever going to get

Origami Fibonacci torus and knotted torus:

I have the impression that the Fibonacci part just gives it a nice organic look (visible in much of Akio Hizume's other architecture at but what interests me is the way it rotates smoothly. That's not something unfolded paper can do, because the inner parts of a torus have negative curvature, the outer parts are positive, and unfolded paper can't change curvature.

Just encountered a really weird firefox bug (I think) where window.setTimeout just never ran.
Refreshing the page didn't work, but closing the tab and reopening it fixed it.
Needless to say, the last 15 minutes have been a trip.

@jsiehler Website:

Specific page:

Questions invited ... answers are either on the website and I can point you to them, or not on the site and need to be added.

And we're off!

Registration is open for the MathsJam weekend & the mass mailing for those on the mailing lists is underway. Cue vast numbers of "Out Of Office" and "User Not Found" emails.

If you're expecting an email & don't get one in the next couple of hours, let me know.

Gergonne's Card Trick, Positional Notation, and Radix Sort
Article by Ethan D. Bolker
In collections: Easily explained, Fun maths facts
Gergonne's three pile card trick has been a favorite of mathematicians for nearly two centuries. This new exposition uses the radix sorting algorithm well known to computer scientists to explain why the trick works, and to explore generalizations. The presentation...
Entry: read.somethingorotherwhatever.

Wonderful mathematical reading:

"Here you can find a variety of mathematical texts on many different topics. One section is related to the “snapshots of modern mathematics from Oberwolfach”, the other section offers general background material connected to our exhibits and projects. We hope you enjoy your read!"

Starting to think that @esoterica needs an opposite category to "Easily Explained" called "Unexpectedly Complicated"

Student asking for 'altercations' following their homework submission.

If it's an altercation they want...

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