Proof Without Words: Gregory’s Theorem | Hacker News Reader

Proof Without Words: Gregory’s Theorem

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My favourite proof without words is the proof that the sum of the interior angles of any triangle is 180 degrees. Proof: draw an arbitrary triangle in chalk on the ground. Stand on one of the lines at one of the corners and note the direction you are facing. Now back up until you are at the corner of the triangle behind you. Rotate your self by shuffling inside the angle to the next line. Walk to the next corner, repeat. Walk (backwards this time) to the last corner, repeat. Now shuffle back to your starting position. You are facing exactly 180 degrees in the other direction!

Of course it is way better to just do this than write it down! But obviously I can't do it on HN.

Does that prove that every triangle has angles adding up to 180, or just that one?

“draw an arbitrary triangle” means this works with any triangle.

But you'd have to try an infinite number of triangles before you were convinced that it worked for all of them.

This is an interesting confusion.

OP said "draw an arbitrary triangle in chalk on the ground", and from there gave a set of instructions that showed you how to check that the interior angles of that triangle added up to 180 degrees. You're right that that concrete set of instructions only proves it for that one triangle.

But that's not the whole proof. If I followed these instructions for a right triangle with angles 45,45,90, I'd only have proved that that triangle's angles added up to 180 degrees.

The full proof is that if you imagine doing this, for any triangle, it's clear that it will always work. When I picture this in my head, it's clear that I'm going to end up having turned 180 degrees, regardless of the measurements of the triangle.

This leap: starting with a set of concrete instructions that you can do on a particular object, but then verifying that they would work no matter what object you started with, is common in proofs.

EDIT: Nevermind, totally unrelated.

Why is it clear that it will always work? It would be a proof if that part was written down. Since we all already know that the angles add up to 180, it might just be the intuition taught to us by another proof of the statement leaking backwards into the implicit step in this attempt at proving the statement. The part that's left off (the proof that it always works) is actually the bigger part of the contraption.

Because you end up back on the line you started at, facing in the opposite direction. That's a 180 degree turn.

Suppose I found a triangle whose internal angles added up to 190 degrees. If I did the experiment on it, I would end up 10 degrees away from where I was predicted to be. How can this scenario be ruled out?

Both of these things are true:

(i) Like you said, if you found a triangle whose internal angles added up to 190 degrees, and you followed the procedure, you would have turned 190 degrees, rather than 180 degrees. This is true because during the procedure, you turned three times, each by one of the angles of the triangle, so the total amount you turned was the sum of the angles.

(ii) You would end up back at the line you started with, facing in exactly the opposite direction. This is true because the last step of the instructions is to turn until you're facing in the direction of this line. Thus you have turned 180 degrees.

Now of course this is nonsense: you can't have turned 180 degrees, but also turned 190 degrees. How did we arrive at nonsense like this? The logic is sound, so it must have been one of the assumptions. Which assumption is questionable? Oh, right, the triangle whose angles added up to 190 degrees.

This is a proof by contradiction, that shows that a triangle whose angles add up to 190 degrees cannot exist.

Inside of that is the assumption that the sum of the inner angles that I rotate by as I walk around a closed loop is equal to the angle between my initial and final directions at the starting point, so that having turned 180 means that my feet have shuffled by a total of 180. That isn't true outside of Euclidean geometry, which indicates that its proof might not be as trivial as it seems.

Bah, I almost talked about what would happen if you did this on a sphere. Yes, there is an assumption that rotations and translations are commutative and associative. We're so used to this that our intuitions sensibly hide it.

The fact that there are hidden assumptions doesn't invalidate the proof, though. There are always hidden assumptions. Even if I give you formal axioms to reason with, you need a system in which to interpret those axioms.

Although I see the reasoning, I'm still not comfortable with the proof. It sounds to me like "plug something in to the Zeta function, observe that it isn't a nontrivial zero, conclude that there are no nontrivial zeros." Even if it seems completely intuitve to me I still wouldn't consider it proven.

Interesting. For me, there is no clear gap between intuitive proofs and formal proofs. Sure, intuition can lead you astray, but as you do mathematics, you develop your intuition so that you stop being intuitively certain of false things. Contrariwise, formal proofs are more likely to contain dumb algebraic errors, but as you do mathematics you learn to be exceedingly careful in your calculations.

But the wider point I want to make is that there's no gap between the two. _Real proofs aren't fully expanded._ If you've worked with a theorem prover like CoQ, it becomes painfully clear how many steps even the simplest proof skips. For example, the proof that the sqrt of 2 is irrational is really easy:

https://www.math.utah.edu/~pa/math/q1.html

But look at how many steps this skips, if you want to get close to actual axioms and definitions:

- You squared both sides of an equation. In this case, that's fine because you're only doing forward reasoning, but if you wanted to reason backwards you'd also have to check that both sides had the same sign to start.

- You multiplied by q^2. That's only valid if q^2 is nonzero. Now intuitively we know that q^2 is nonzero, since q is nonzero. But it needs a proof.

- You deduced that p was even from the fact that p^2 was even. How do you prove that? My first thought is to use the fundamental theorem of arithmetic. I don't know about you, but my intuition completely glossed over the fact that the proof that sqrt(2) is irrational made use of the fundamental theorem of arithmetic when I first read it. Either that, or there's another way to prove this; what is it?

Now, you could expand this proof to include all the steps. But we don't bother, because it's painstaking and not actually that likely to catch flaws in the proof, because we intuitively know these things. Likewise, I feel like the triangle proof is the same way: it skips over some things, but we intuitively know it's okay and you could expand it (to talk about commutativity of translation and rotation) but there's usually not much reason to bother.

Although it's a lot more obvious how to go about expanding the proof that sqrt(2) is irrational, I'll give you that.

There is an implicit, widely-held sense that the detail in proofs should scale with the expected training of the people who will be reading them. If your intended audience has internalized their field so well that the expansion and checking happens completely automatically and subconsciously, more power to them - but on the other end of the scale you have the proof that the square root of two is irrational, or this one about interior angles. Other than machine-aided proofs the most thoroughly expanded you ever see anything is in highschool geometry!

So, what justifies this scheme? I would say that once you have seen a technique used in full detail, you don't need to see the detail elsewhere because there's a meta-theorem in your head that applies to every case where things line up in a pattern where the technique works. Slowly this replaces your natural intuition.

I haven't previously come across the idea of trying to make wordless proofs. I guess this is the mathematical equivalent of code golf: a clever game for insiders to play, but basically the opposite of good practice from a readability perspective.

I didn’t get this feeling from the proof in the article. I thought there was a kind of poetry to it. The diagram and colors make it reasonably easy to understand and they emphasize the geometrical nature of the proof, which uses only elementary math: the formula for the area of a triangle and Thales’ theorem (If my memory is correct, I haven’t used that name since high school!).

On the other hand, a “proof with words” would likely have derived the explicit formulas for I_n and C_n in order to prove the relationship and would have been much more clanky.

If I had to make a comparison, this felt like reading good functional code. Concise, a bit mysterious at first, but once you understand what it does you feel enlightened. (Vs eg. wading through for loops and index arithmetic, which also does the job but is more tedious to read)

I think it wasn't so much a proof as it was an image that made you realize the proof with some thought. Still, that's about what all published proofs are in the end, isn't it?

Same for education purposes, I deeply believe that most terms cloud the learning process. Most of the time a pattern can be witnessed and learned without giving too many names if at all. Maybe then, after the structure/topology[1] has set in the mind, you can label things to accelerate communication. But words first feels like cart before the wheel.

[1] in the fuzzy sense, not the mathematical field

>I haven't previously come across the idea of trying to make wordless proofs. I guess this is the mathematical equivalent of code golf: a clever game for insiders to play, but basically the opposite of good practice from a readability perspective.

There are lots of geometrical proofs for e.g. the pythagorean theorem that need no words. It's not like code golf any more than a text-based proof is.

Words still need to get interpreted just as images do.

If anything, a succinct geometric proof can be much easier to grasp and verify than a text-based proof.

> a clever game for insiders to play, but basically the opposite of good practice from a readability perspective.

This is truly, deeply the wrong lesson to take away.

I'd say that, in a spirit similar to the Feynman technique, you don't really understand proofs whose central idea you can't explain simply -- and a wordless proof is good evidence that you have in fact grasped the proof's essence.

Of course, this doesn't mean you can do away with formal rigor. Mathematics needs that too. But I'd argue that is "the clever game for insiders to play" (albeit an important game), while intuitive understanding is what you should really be after, and is what generates new ideas and new proofs.

More beautiful proofs without words:

https://math.stackexchange.com/questions/733754/visually-stu...

https://math.stackexchange.com/questions/1337750/easy-math-p...

https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3...

Here is another proof without words for the Pythagorean Theorem: https://en.wikipedia.org/wiki/Pythagorean_theorem#/media/Fil...

Nice. I also like the proof which uses the only the first image a calculates the area of the square twice:

    c^2 + 4*a*b/2 = c^2 + 2*a*b

    (a+b)^2 = a^2 + b^2 + 2*a*b

Evidently Oliver Selfridge [1] said a puzzle based on deriving the radius like this once tripped Feynman up...

"Puzzles from last week" http://web.media.mit.edu/~walter/MAS-A12/week11.html

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

What's the expected mistaken answer?

Hi there. Just a precision: the fact that the sum of the interior angles of a triangle is 180 degrees is not really provable, it is a postulate of the Euclidean geometry. It is an intuitive and accepted property, it reflects the fact that the geometric space usually considered is flat. Proving this statement would be possible provided that you change basic postulates. At that moment, it would become a proposition of these new mathematics and would certainly be provable starting from the new postulates. Changing these postulates is possible, that's how we got the parabolic and hyperbolic geometries, see wiki!

great. now draw a few hundred more n. and don't forget the min-max values.

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