Live data from Hacker News

Discovery of a new irregular pentagon that can cover the plane

theguardian.com

11–20 of 131 posts

Re: Discovery of a new irregular pentagon that can cover the plane

#11
post #10

I think it's kinda funny that most of the tessellations are just using pentagons to make other shapes that tessellate naturally. I suppose the same could be said of most tessellations though, but it's still interesting.

That's because there are only 17 wallpaper groups (https://en.m.wikipedia.org/wiki/Wallpaper_group). Any _repeating_ pattern must match one of them (non-repeating patterns by definition do not)

Re: Discovery of a new irregular pentagon that can cover the plane

#12
post #6

Earlier quoted context omitted.

Yeah, great reasons why it doesn't work—what 5-sided polygons are you going to run through? The hard bit is the side lengths; there are good reasons to think that the angles cannot be too weird. But for example, this pentagon has one side of side length 1 / (sqrt(2) (sqrt(3) - 1)). How long is your exhaustive enumeration going to go until it finds that one? Anyway, it sounds like clever enumeration is exactly what th…

> 1 / (sqrt(2) (sqrt(3) - 1)) I don't mean in any way to demean their research, but I think your exhaustive search could build up formulas in exactly that form and iterate on them. It starts with a valid pentagon. Then it permutes that pentagon with an evolutionary method by modifying the formula. For example, maybe the lengths it tries are: 1 / 2 1 / sqrt(2) 1 / sqrt(2) * 3 1 / sqrt(2) - 1 1 / sqrt(2) * (3) 1 / sqrt…

"I'm sure there's way more complexity I'm overlooking, but that's how one might get started."

That's how I'd start. There's probably a way to make the search a lot smarter. Here's the part you're overlooking, though:

"... there are only so [many] formulas made up of a fixed number of terms and operators..."

"So many" = "countably infinite." Paring it down to finitely many would require understanding tantamount to having solved the problem in the first place.

[Edit: I can words.]

Re: Discovery of a new irregular pentagon that can cover the plane

#13
post #6

Earlier quoted context omitted.

Yeah, great reasons why it doesn't work—what 5-sided polygons are you going to run through? The hard bit is the side lengths; there are good reasons to think that the angles cannot be too weird. But for example, this pentagon has one side of side length 1 / (sqrt(2) (sqrt(3) - 1)). How long is your exhaustive enumeration going to go until it finds that one? Anyway, it sounds like clever enumeration is exactly what th…

> 1 / (sqrt(2) (sqrt(3) - 1)) I don't mean in any way to demean their research, but I think your exhaustive search could build up formulas in exactly that form and iterate on them. It starts with a valid pentagon. Then it permutes that pentagon with an evolutionary method by modifying the formula. For example, maybe the lengths it tries are: 1 / 2 1 / sqrt(2) 1 / sqrt(2) * 3 1 / sqrt(2) - 1 1 / sqrt(2) * (3) 1 / sqrt…

This could work for approximations. But if you want the exact value, this might not be super effective, as there are values without closed forms (that is to say, that are not expressible simply with the *-+/ and sqrt/log and other usual operations).

So your iteration procedure will probably get to a good approximation, but might not find the "core" expression.

Re: Discovery of a new irregular pentagon that can cover the plane

#14
post #5

"That same year an unlikely mathematical pioneer entered the fray: Marjorie Rice, a San Diego housewife in her 50s, who had read about James’ discovery in Scientific American. An amateur mathematician, Rice developed her own notation and method and over the next few years discovered another four types of pentagon that tile the plane. " https://en.wikipedia.org/wiki/Marjorie_Rice

Rice's pentagons are illustrated here [1]: Type 9, Type 11, Type 12 and Type 13.

[1] http://www.mathpuzzle.com/tilepent.html

Re: Discovery of a new irregular pentagon that can cover the plane

#15

This is cool, but I'm not sure why it's a hard problem to solve for a computer running through a ton of 5-sided polygons with some basic rules and attempting to tessellate them? Is there a reason that approach doesn't work, other than being rather un-romantic as far as discovering cool new things like this goes?

Yeah, great reasons why it doesn't work—what 5-sided polygons are you going to run through? The hard bit is the side lengths; there are good reasons to think that the angles cannot be too weird. But for example, this pentagon has one side of side length 1 / (sqrt(2) (sqrt(3) - 1)). How long is your exhaustive enumeration going to go until it finds that one? Anyway, it sounds like clever enumeration is exactly what th…

mind elaborating on why the angles can't be too weird? I thought it was odd that the angles given were all integers.

Re: Discovery of a new irregular pentagon that can cover the plane

#17
post #15

Earlier quoted context omitted.

Yeah, great reasons why it doesn't work—what 5-sided polygons are you going to run through? The hard bit is the side lengths; there are good reasons to think that the angles cannot be too weird. But for example, this pentagon has one side of side length 1 / (sqrt(2) (sqrt(3) - 1)). How long is your exhaustive enumeration going to go until it finds that one? Anyway, it sounds like clever enumeration is exactly what th…

mind elaborating on why the angles can't be too weird? I thought it was odd that the angles given were all integers.

The one thing that the unit of "degrees" has going for it is that it tends to give integer answers (because 360 is so divisible). If you convert them into rotations around a circle, they angles become (1/6, 3/8, 7/24, 1/4, 5/12). (Multyply those by τ=2π to get the angle in radians).

EDIT:

To your question as to why angles can't be to weird, consider vertices, where the corners of the pentagons meet. Each vertex is composed of three angles, one from each of the three neighboring vertices (although it is not a-priori obvious that a vertex contains only three angles). The sum of these angles (in rotations) must be 1. This means that, heuritstically, you would want as many permutations of the angles as possible to add up to one.

Re: Discovery of a new irregular pentagon that can cover the plane

#19
post #6

Earlier quoted context omitted.

Yeah, great reasons why it doesn't work—what 5-sided polygons are you going to run through? The hard bit is the side lengths; there are good reasons to think that the angles cannot be too weird. But for example, this pentagon has one side of side length 1 / (sqrt(2) (sqrt(3) - 1)). How long is your exhaustive enumeration going to go until it finds that one? Anyway, it sounds like clever enumeration is exactly what th…

> 1 / (sqrt(2) (sqrt(3) - 1)) I don't mean in any way to demean their research, but I think your exhaustive search could build up formulas in exactly that form and iterate on them. It starts with a valid pentagon. Then it permutes that pentagon with an evolutionary method by modifying the formula. For example, maybe the lengths it tries are: 1 / 2 1 / sqrt(2) 1 / sqrt(2) * 3 1 / sqrt(2) - 1 1 / sqrt(2) * (3) 1 / sqrt…

In this case, (with the hindsight of knowing the answer), it would probably be easier to enumerate the angles, as they were all rational multiples of pi (with the biggest denominator being 12). Still, that gives a naive 5^144 combinations of angles to test. If you reduce this to proper, reduced fractions this comes down to 5^47 [1]. You can probably improve on this further by requiring that the angles form a (convex) polygon, but I suspect that still leaves you with to many to be practical.

[1] http://oeis.org/A005728

Post reply on HN