Bourbaki dangerous bend symbol | Hacker News Reader

Bourbaki dangerous bend symbol

en.wikipedia.org

129 pointszerojames60 comments

I put the symbol on my shell prompt line, when there's a readme file found in the current directory. Just a reminder, especially for directories, in which I'd put the readme file myself, but then forgot about it.

https://github.com/rybak/scripts/blob/master/config/ps1_noti...

It works together with very dumb script in $PATH, which prints a readme file if one is found in the current directory.

https://github.com/rybak/scripts/blob/master/commands/readme

That is pretty brilliant. Nicely done.

Somewhat related: the “slippery when wet” road sign used in Australia and New Zealand <https://c.stocksy.com/a/jLM400/z9/1039227.jpg> doesn’t actually depict two tyre treads crossing over in a physically impossible way, but one tyre tread and the edge of the road. I finally realised this for no apparent reason at the age of 29 when cycling through New Zealand. None of my family had realised this, though we’d certainly talked about the signs before: we’d all just just written it off as a weird design.

I looked at the image before reading the rest of your comment. My first thought was that the tire tracks crossing each other was supposed to indicate a full rotation of the car. (It’s really slippery, you might spin out!)

After reading your comment I still don’t think one of the lines is a road’s edge?

Do you know if either interpretation is the official one?

EDIT: Just took another look and I see it now. The line near the car’s left tire doesn’t actually align with that tire!

The dramatically spinning car is how I read the sign, especially combined with what appears to be a straight road in the background (although we can't tell if there's a bend afterwards it looks like extremely straight)

Easy to clear up (by Australian/NZ folks), do these show up on straightaways? Are they mirrored to indicate which direction the bend is going to be?

> The line near the car’s left tire doesn’t actually align with that tire!

could be:

- pointing to and suggesting the other wheel on the right side, more like materialising an envelope of the skid marks instead of realistic skid marks themselves

- stylisation of the overall 180 rotation, emphasising the overall spin out of control movement

(NZ here) The signs don't get mirrored. They could show up on straight roads, but are much more common in places with corners.

I've always interpreted it as a cartoony car version of what your legs do when slipping on ice - sliding out from under you in random directions.

The sign makes a lot more sense when you view it this way, even the message is clearer: It's not just warning that you could generally lose control of your car but specifically that you could drive off the road because of that.

But yeah, the design makes this very hard to recognise and suggests more the "two crossed tire lines" interpretation which gives this a cartoonish, tongue-in-cheek vibe, which is completely unfitting.

Good idea, bad execution I think.

The UK has a similar sign. I always read it as the wheels crossing too -- definitely makes much more sense that it’s the kerb!

I think the cartoonish feel actually helps, though, as it makes it more memorable.

Similar design on the french version of this sign, but the lines do not cross. https://fr.m.wikipedia.org/wiki/Panneau_de_signalisation_de_...

Ironically I think this change in design makes it actually impossible, since now both lines appear to be tyre treads and they still come too close together.

The treads can come close together if the rear wheels lock up and the car twists/drifts, but the pattern in the sign is still “practically” impossible. Eg I can’t think of any movement a car could realistically do to make that pattern except maybe if the driver pulled the handbrake on then the car was buffeted by a tornado in random directions.

We don’t know the perspective or the contours of the illustrated road well enough to say this. I could imagine some perspectives where this actually works out, especially if the tires are causing the vehicle to slide sideways.

Isn't "slippery when wet" just a reminder for people in dry climates ?

When it rains anywhere, the road is always slippery for the first half hour or so because of accumulated car gunk that has to wash off. So if it rains infrequently and briefly, it's not much of an overstatement to flatly say "slippery when wet".

UK has these signs everywhere

A reminder for people in wet climates too.

In arid climates, heavily-used roads end up with oily residue on them, and then the first little bit of rain basically makes the road into Spy Hunter[1] oil slicks. The road actually gets less slippery after the first 30 minutes of rain, as the oils get rinsed off.

[1]: https://en.wikipedia.org/wiki/Spy_Hunter

I thought it was to emphasize how extremely slippery it may be, in a humorous manner, that even your tyres would cross! Much more striking that way IMO.

Fittingly, it’s right between U+2620 SKULL AND CROSSBONES and U+2622 RADIOACTIVE SIGN.

And amazingly it was a part of the original Unicode 1.0.

I'd put a "dangerous bend" symbol in front of the "dangerous bend" symbol itself - for authors: It's such an interesting and unusual tool that authors who have it in their arsenal will want to use it, even if just to show off some interesting bit of typography in their book and possibly interrupt the monotonicity of an otherwise very strict formal textbook.

But this means that the symbol can excuse, or even incentivize writing exactly such "dangerous bend" passages instead of e.g. structuring the argument or the learning order differently, so the thinking trap doesn't occur in the first place.

I think it' a very useful didactic technique to show common misconceptions of something, or popular simplistic but wrong interpretations of a concept. But then it's important to explain what exactly the misconception is and why it is wrong, and not just put a "beware, here be misconceptions" sign next to the introduction of the concept.

Another article on a bit of mathematical typography linked from that page, which made me double-check that the fonts were working properly:

https://en.wikipedia.org/wiki/Tombstone_(typography)

Just a buncha boxes. Where do those Mathies come up with such creativity?

(disclaimer: am son of two Mathies who met in a combinatorics lecture)

The discrete product of a discreet permutation then?

The Wikipedia article says that it was meant to serve the same purpose as end boxes in magazines, and I think it is kind of creative to repurpose that same typographical element for math proofs.

In French, we use the expression: "Sortie de route" (going off the road) when someone says something awkward or terribly wrong. Basically, the real meaning of this expression is that you lost the control of your car and went off road. We also say "partir dans le décor" (to go into the scenery), which is used in the same context. And usually you loose control when you take a curve to fast. Hence this symbol.

In English you can go off the road in a metaphorical way too

Are you in Mathia yourself?

At first it might look a bit pretentious: "Ok thickie, this will hurt your ordinary mind". However, on reflection, a warning that the writer has bothered to put their back into the job for a while is a great idea. When you see it, stop, put on the kettle, go out for a fag (cigarette) or whatever and then come back.

Although Bourbaki was/were capable of being plenty pretentious, I think that it might seem less pretentious if read not as "from our lofty viewpoint, we can see that this is too tough for you," but rather as a very literal analogue of the road sign: "it's easy to get dangerous results if you traverse this territory without sufficient care."

I was taking the piss 8) I will definitely be deploying this symbol in some of my documentation.

Given I'm an IT bod: it will be the ones pertaining to name resolution and ... time.

> I will definitely be deploying this symbol in some of my documentation.

+1. Also curious to see where it should be positioned on the page. The wiki page says it goes in the margin. I tried searching github for 'u2621' to find examples but couldn't spot any rendered examples yet (only code).

Check out the TeXBook, eg. page 14.

Yeah, other possible reads:

- "brace yourself, here be dragons"

- "til now it was a bit of a treadmill, but that's a part you probably don't want to fast read"

- "on the surface this may look innocuous but there are tricky subtleties that you may not want to overlook"

  /*
   * You are not expected to understand this.
   */

why the addition to clarify your use of fag?

The word fag has multiple meanings across the various en_* variants, including cigarette and (a) gay.

Many readers hereabouts read English as a second language. It took me roughly two seconds to ensure that my meaning was spelled out. This response, rather longer 8)

Related: Knuth’s collection of diamond road signs.

https://www-cs-faculty.stanford.edu/~knuth/diamondsigns/diam...

I'm a bit sad he doesn't have a snake crossing sign (lots of snakes breed in the coulees).

https://maps.app.goo.gl/6ZXzKGNvQBw3mbmK6

This reminds me of how the "For Dummies" books would always have a few symbols to mark off certain sections of the book. Quite useful, actually.

The sign seems confusing to me; just looks like a lopsided Z. I much prefer the American design: https://www.shutterstock.com/image-photo/curvy-road-sign-mou...

Note that this was the French design before 1949. Current designs have arrows, like the American design: https://w7.pngwing.com/pngs/369/604/png-transparent-traffic-...

> current designs

They don't ? (well, not a full arrowhead, but wedged head and tail)

https://evs-profilepics.s3.amazonaws.com/production/cdr/imag...

(The important bit being that it indicates which direction is the first turn going to be)

Sadly it's not named after the Nickieben Bourbaki who lives at 1 River Road in West Newbury, and spent a year working on a cattle ranch in Wyoming[1].

1: https://dreamsongs.com/Nickieben.html

In both contexts, I read this as “fun ahead!”.

How mathematicians use it:

> to mark passages tricky on a first reading or with an especially difficult argument.

When I try this (on macOS) echo -e "\u26A0" it gives the 'triangle with an exclamation mark in it' icon (i.e. https://imgur.com/a/SJbl67s).

I think there's no exact unicode equivalent of the dangerous bend symbol?

Shouldn’t that be \u2621 ? On mobile so I can’t test it myself atm

Gahh.. you're spot on! Ty!

https://imgur.com/a/phiJZkP

Note that at least Samsung renders the Unicode character as a yellow warning triangle, because “caution sign” I guess: https://emojipedia.org/samsung/one-ui-6.0/caution-sign

I gave a couple of physics talks where I used the "fasten seatbelt" sign on a series of slides introducing a controversial topic (weak value amplification). I told the audiences beforehand that they just needed to stay in their seats for these slides and we'd discuss once I'd laid everything out.

Really looks like the Russian Z. I thought that's why it was posted at first. Interesting nonetheless.

To be clear, Russian alphabet does not have a 'Z'. But I think I know what you mean.

I am referring to the Z that was alongside the V and O painted onto the tanks and other heavy military vehicles that invaded Ukraine, referring (presumably) to Volodymyr Oleksandrovych Zelenskyy, as a means to gaslight the Ukrainian people whilst they were slaughtered.

I'm fully aware Russia does not have a literal 'Z' glyph.

Most notably, this Wikipedia page hasn’t been updated in over two years!

It’s a fun little game I have with a circle of text friends. Almost everything you see there was updated last week. Sometimes you’ll find something that was updated a few months ago. Two years is epic.

I'm coming across this term for the first time and think it will be very useful. The most obvious thing missing from the wiki page is some visual examples of its usage. The wiki says it "appears in the margins of mathematics books" but would be great to see some examples.

MathCast a few hops across is over four years: https://en.wikipedia.org/wiki/MathCast

Ha, my equivalent before a galaxy-brained code comment is

// Hold on to your butts...

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