ℝolliℵg M∀th Thr∑a∂

In mod 7 arithmetic, dividing by 10 is the equivalent multiplying by 5 is the gist of it.

Basically this is a further generalization of the principle that makes the divisibility tests for 3,9 and 11 work, except none of us have ever seen it before

Well, yeah, but the "multiply last digit by 2 and subtract from the rest" test for divisibility by 7 is well-known, and that test is to 21 as Chika's test is to 49.

Didn’t remember that one but -2 is 5 mod 7 so it makes sense.

Arguably subtraction is harder to do mentally than addition, so Chika’s method seems easier.

Fair enough. Although multiplication by two and then subtracting a really small number is (slightly) easier than multiplying by five and adding a larger number.

It’s kind of fun once you know this trick to come up with other rules for checking for divisibility.

For example to check for divisibility by 29, you can take the last digit, multiply by 3, add to remaining part of the number, and just keep doing that until you get to 29 or a number which is less than 29.

RIght. I never knew the trick for divisibility by 13 before last night and of course it fits the same pattern.

Multiply last digit by 4 and add, because 39.

Yup

When I was 15 I did this— it was so funny watching them be mad, claiming I was a computer or cheating. Nope, just a genius.

Small world I see this on twitter.
pic.twitter.com/rQtxX3eoaq

— @chloe21e8灭绝公主 (@chloe21e8) April 21, 2023

posting quean

!

Do we know whether she was also posting the questions? Seems perhaps a bit too convenient that people just happened to ask about some rather contrived integrals that just happened to have neat and tidy solutions that one poster knew the answer to.

Certainly seems possible.

i enjoyed this thread about zero:

This meme is very very funny, but it is also inviting a serious explainer 🧵 from yours truly about 0 in Sinitic languages (there'll also be something funny later on). 😂 Let's go! 1/ https://t.co/I3vEGnNS7Z

— Egas Moniz-Bandeira ᠡᡤᠠᠰ ᠮᠣᠨᠢᠰ ᠪᠠᠨᡩ᠋ᠠᠶᠢᠷᠠ (@egasmb) May 21, 2023

also have enjoyed videos recently from math youtuber @Domotro

That was good, thanks.

as solved by "anonymous 4chan poster and anime-fan":

https://www.quantamagazine.org/sci-fi-writer-greg-egan-and-anonymous-math-whiz-advance-permutation-problem-20181105/

(courtesy max read so others will have seen this)

The Unabomber has died. Here is a famously good footnote to him in a mathematics paper.

Citation: Pudwell, Lara. “Digit Reversal without Apology.” Mathematics Magazine, vol. 80, no. 2, 2007, pp. 129–32. JSTOR, https://t.co/IpyzPWc0AA. Accessed 10 June 2023. pic.twitter.com/xCwNbMtQmf

— Alex von Tunzelmann (@alexvtunzelmann) June 10, 2023

i love Greg Egan's short stories. he is better versed in math than more recognized literary writers like Borges, Calvino, et al

Subscribers to this thread might enjoy the novella a short stay in hell by this guy https://en.wikipedia.org/wiki/Steven_L._Peck

There’s something annoying about Greg Egan but some of his stories are indeed mindblowing.

https://www.youtube.com/watch?v=ArADlJx7SlU
I love that she has a Penrose tiling tattoo.

https://www.youtube.com/watch?v=_ZS3Oqg1AX0

https://cs.uwaterloo.ca/~csk/hat/

https://www.youtube.com/watch?v=FUHkTs-Ipfg

thought this bump would be about terry tao's formalized proof of the Polynomial Freiman-Ruzsa conjecture using Lean4 proof checking language

https://terrytao.wordpress.com/2023/11/18/formalizing-the-proof-of-pfr-in-lean4-using-blueprint-a-short-tour/

(this post was written while it was still ongoing but they finished it the other day)

super interesting stuff. i don't think s clover still posts but would be interested to hear his thoughts on the significance of this. seems like it was cute niche a few years ago but is now catching on

I watched that coin rotation video the other day, pretty cool - especially solar vs sidereal year.