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Comment by firstcommentyo | Hacker News Reader

Dislosure: I'm not directly from the fields of the Sciences Of Angles And Ambiguously Crossing Lines nor I've every seen or used this symbol before. However to me it's, pretty evidently, supposed to be a "no right angle" symbol.

(A) It's in the math section, (B) it's with angles, (C) the thunderbolt ↯ is commonly used for "not" or more specifically for dis-proof in this area and

(D) at least by my 30 s internet search on a mobile phone I couldn't find any other "no-angle" or "no-right-angle" symbol.

Someone could argue that usually you use a simple strike through as like as in ≠ (unequal), ∉ (not-element-of) or ∅ (empty set) but I would say it was chosen to avoid confusion in this case. The angle itself (without the "no/not") consists of only to orthogonal lines so it would be kinda complicated to "strike it though" in any direction without ambiguity that would resemble a triangle, a fork or whatnot.

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> the thunderbolt ↯ is commonly used for "not" or more specifically for dis-proof in this area and

I don't think it's that common. At least, I don't recall seeing it ever. Maybe it's used in non-English mathematics?

Wikipedia mentions it's also used in electrolysis so maybe this new one is related to that somehow?

It's used in german mathematics education (secondary level), either to mark a contradiction in a proof or more generally to mark an erroneous statement.

Also in Dutch universities to mark a contradiction, especially in a proof by contradiction.

But I have never seen it to mark negation of a condition, that's usually done with a slash (as in ≠ ≮ ≯ ≰ ≱ ≴ ≵ ⊄ ⊅ ⊈ ⊉ ⊊ ⊋ ∉ ∌ ∄ ∦, you get the idea).

So for "not a right angle" I'd have expected a "right angle" symbol with a slash through it.

But how would you position the slash to get a somewhat easy to decipher symbol? To me, the right angle symbol seems to lend itself more to this unorthodox negation through the contradiction symbol than to negation through the normal slash.

Funny enough, I've only seen it at the Gymnasium (secondary level) and not in the University a few years later -- then indeed the usual symbols were the 'slashed' relations like you've described, or the bottom symbol: ⊥ in logic. Maybe it's an idiosyncrasy of a certain subset of math teachers.

Same. Never seen that symbol in my life. I've seen ¬, ~, !, etc used for not/negation in computer science, math, logic, etc.

And some commenters said they used it to mark proof by contradiction, but why is there a need to mark it when you are showing it via proof? A canonical example of proof by contradiction is proving sqrt(2) is not rational. Never have I seen it marked with that symbol. Where would you even mark it? At the beginning with the assumption? Or at the end like QED?

Math degree holder from Iowa, yeah, I've seen and used it many times. The symbol is used when you reach the contradictory statement. Like "1 = 2".

"By way of contradiction suppose P, then ..., thus ~P ↯. Therefore ..."

To me that reads: I throw your assumption into the GROUND!

I've actually always wanted a way to mark "the contradiction" once I've obtained it. Thanks!

I was taught it in extracurricular mathematics in Australia. We were taught that it goes at the end of a contradiction proof once the contradiction has been found. We used to write it extra large, like lightning strike. I think of it like a proof mic-drop.

I believe I have seen it used as a symbol which indicates the discharge of an assumption, but never for "not".

It's the first symbol referenced for symbols used in proof by contradiction to show contradiction [0]. I know that's not exactly "not" or "disproof" but I think that might be what the poster was getting at.

[0] https://en.wikipedia.org/wiki/Contradiction#Symbolic_represe...

I submit to you that it's clearly not a thunderbolt but an arrow indicating changing directions; that being overlaid on top of a pair of axes is obviously useful in the study of non-Euclidean geometry to indicate the use of wibbly-wobbly dimensions.

Particularly useful for timey-whimey relativistic analyses.

Related to The Whole Sort Of General Mish Mash.

I've thought that it would be cool to have a Wiki with an entry for each character, describing what it is, and its history. Although that wouldn't help for mystery characters like this one, there are a lot of characters with stories behind them.

I like this idea. It would serve as a place to put a well-sourced answer to the question about this character, and the talk section could be used to discuss further investigation into the topic, or when new uses inevitably arise.

I was just discussing :man-in-business-suit-levitating: with some friends earlier today. Also an interestingly cryptic background, albeit not an unsolved one.

https://emojipedia.org/person-in-suit-levitating/

(Edit: Apparently HN automatically removes emoji.)

The story behind MIBSL is definitely fascinating and some great trivia there. There’s a longer article about it here: https://www.newsweek.com/2016/05/06/secret-ska-history-man-b... that covers not just the inspiration for the emoji itself, but a brief history behind the inspiration behind the inspiration. Lots of levels of metaness to unpack.

I love this! I've always assumed it was a rude boy emoji. Was briefly in a high school ska band :D

Wikipedia already does this for many symbols. See for example...

https://en.wikipedia.org/wiki/Miscellaneous_Technical

Aside from the table describing each symbol, if you scroll to the bottom of the page, it links out to full articles related to each. For a full list see...

https://en.wikipedia.org/wiki/List_of_Unicode_characters

That doesn't jive with the history in TFA — the Unicode name and location was inferred from the symbol itself without knowledge of its meaning.

I don't see the contradiction. The only thing they used from the name is the "right angle" aspect. Given their argument is this is a composition of thunderbolt + X, for some X (and derived from their prior knowledge of thunderbolt's compositional meaning), deciphering the image as "thunderbolt + right angle" is trivial and consistent with the naming origin in TFA.

Right angles have a small box near the vertex which denotes it is a right angle [0].

This symbol doesn't have that box, so I don't think it's a right angle.

[0] https://en.wikipedia.org/wiki/Right_angle#/media/File:Right_...

Edit: This merely adds to the confusion, since the name of the glyph contains the words "right angle."

¯\_(ツ)_/¯

https://en.wikipedia.org/wiki/Right_angle

> In Unicode, the symbol for a right angle is U+221F ∟ RIGHT ANGLE (HTML ∟ · &angrt;). It should not be confused with the similarly shaped symbol U+231E ⌞ BOTTOM LEFT CORNER (HTML ⌞ · &dlcorn;, &llcorner;). Related symbols are U+22BE ⊾ RIGHT ANGLE WITH ARC (HTML ⊾ · &angrtvb;), U+299C ⦜ RIGHT ANGLE VARIANT WITH SQUARE (HTML ⦜ · &vangrt;), and U+299D ⦝ MEASURED RIGHT ANGLE WITH DOT (HTML ⦝ · &angrtvbd;).[5]

> In diagrams, the fact that an angle is a right angle is usually expressed by adding a small right angle that forms a square with the angle in the diagram, as seen in the diagram of a right triangle (in British English, a right-angled triangle) to the right. The symbol for a measured angle, an arc, with a dot, is used in some European countries, including German-speaking countries and Poland, as an alternative symbol for a right angle.[6]

This merely adds to the confusion, since the name of the glyph contains the words "right angle."

The article notes that sans a given meaning the glyph was given a “descriptive name”.

So you’re not wrong? :-P

Perpendicular + Unicode combining solidus = ⟂ + / = ⟂̸

I think perpendicular most commonly refers to lines/vectors/planes etc., while the right angle symbol refers to angles. Also, there are often multiple symbols expressing the same thing.

Yes, a sibling comment meanwhile found the right angle symbol U+221F ∟

https://news.ycombinator.com/item?id=31015104

> (C) the thunderbolt ↯ is commonly used for "not" or more specifically for dis-proof in this area

Any examples?

High school physics and math as a major. I could scan you my scripts and papers if you're interested.....no won't. ;-D

But maybe "commonly used" was maybe the wrong term. More appropriately: "sometimes" or "by some".

I have never seen it used as not once in maths or physics. "extremely rarely" perhaps.

To be fair, there are lot of math symbols out there.

http://mirrors.dotsrc.org/ctan/info/symbols/comprehensive/sy...

There are lots of examples of the lightning bolt in there. In fact, under ulsy Contradiction symbols, there are four variants.

I also noticed the exact symbol being discussed is listed under "Angles".

Where in the world? I’ve never used it despite similar background. Perhaps regional?

I believe in German (possibly also other languages) the thunderbolt ↯ is commonly used to mean "this is a contradiction" in a mathematical proof, equivalently to in English a kind of ⋕ rotated by 45° or the symbol ※. The symbol ⟂ on the other hand means "false" and is used in particular in formal logic.

Yes and no.

Yes, we indeed used and afaict still use the thunderbolt for contradiction in my German university.

However "perpendicular" and "bottom/falsum" are two different Unicode codepoints with very similar glyphs.

https://en.m.wikipedia.org/wiki/Up_tack

I've seen it used for contradiction. Though that's not the same thing as 'not' and I can't think of why you'd combine this with orthogonality.

If the thunderbolt means not, and the right angle is displaying the x and y axis, then this symbol could be a pun for "not a function".

Reminds me of https://xkcd.com/2606/

Explained: https://www.explainxkcd.com/wiki/index.php/2606

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