## Circles on a plane

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Kerberos
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### Circles on a plane

I have a plane that I want to cover with circles. Circles can overlap, but every point on the plane must be inside at least one circle.

How do I achieve this with the smallest number of circles? That is to say, how can I do this with the least overlap between them? I have the suspicion that
Spoiler:
each circle intersects six other circles, and every intersection is between three circles. Like this:

You might be able to tell that I don't have any software that lets me draw overlapping circles.

But I'm not sure.

EdgarJPublius
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### Re: Circles on a plane

can you not just
Spoiler:
use one arbitrarily large circle?
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Govalant
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### Re: Circles on a plane

That's it, overlap 6.

Cover the plane with hexagons. The hexagons conected end-on-end should cover everything. Now draw the circle on each hexagon, from the center of the hexagon to one the vertices.
Now these points of data make a beautiful line.

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Kerberos
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### Re: Circles on a plane

EdgarJPublius wrote:can you not just
Spoiler:
use one arbitrarily large circle?

Er... I forgot to mention that all the circles are of the same, finite size.

And Govalent, have you a proof?

bittyx
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### Re: Circles on a plane

Everything from here up is useless and shouldn't have been included. -G

3) I would agree that the hexagon thing works. Regular tilings are triangular, square and hexagonal. It would be easy to just check each one and see which overlaps the least, but, instinctively, I would say the circumscribed circles of hexagons are the best choice.

Token
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### Re: Circles on a plane

EdgarJPublius wrote:can you not just
Spoiler:
use one arbitrarily large circle?

You can't have a single arbitrarily large circle. It makes no sense. Either it's finite size, or it's not a circle. "Arbitrarily large" generally applies to sets where the size of individual elements is unbounded. It can't really be used to talk about a single, finite object like a circle.
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Torn Apart By Dingos
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### Re: Circles on a plane

How do you define "least overlap"? All coverings will have an overlap that is infinite, of the same cardinality.

One way is to measure it as the limit of the quotient of overlap of coverings of finite regions, letting the regions tend to the whole plane, but I'm not sure you'd get the same result no matter how you choose the regions (though it seems plausible).

Robin S
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### Re: Circles on a plane

You would as long as the shapes of the regions were reasonably sensible (if you actively tried to find a set of regions of increasing size whose limit for the quotient was anomalous, you definitely could) - unless perhaps it was a particularly nasty sort of irregular tiling, which seems to me unlikely to be optimal.
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Token
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### Re: Circles on a plane

Torn Apart By Dingos wrote:How do you define "least overlap"? All coverings will have an overlap that is infinite, of the same cardinality.

One way is to measure it as the limit of the quotient of overlap of coverings of finite regions, letting the regions tend to the whole plane, but I'm not sure you'd get the same result no matter how you choose the regions (though it seems plausible).

I imagine you would do it in the same way that you define what percentage of the plane a non-overlapping tiling covers.
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Torn Apart By Dingos
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### Re: Circles on a plane

It'd be helpful if you said how that is done. I assume it's defined as the ratio of overlap of a single tile. But a circle covering of the plane needn't be tileable, so I don't know how it could be generalized.

Owehn
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### Re: Circles on a plane

I suppose you could define it by taking a convex, absorbent, measurable subset A of the plane (like a circle, or a nondegenerate rectangle), and then for each natural number n check to see what the fraction of area nA has that consists of overlap in the circles. You want to minimize the limit of this fraction as n approaches infinity, assuming that it is well-defined.

Last edited by Owehn on Sat Oct 27, 2007 8:41 pm UTC, edited 1 time in total.
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Token
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### Re: Circles on a plane

Torn Apart By Dingos wrote:It'd be helpful if you said how that is done. I assume it's defined as the ratio of overlap of a single tile. But a circle covering of the plane needn't be tileable, so I don't know how it could be generalized.

Well, in the case that the packing is periodic, it's obvious. If it isn't, then I don't actually know if you can even calculate it.
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jestingrabbit
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### Re: Circles on a plane

The translation symmetries of the plane are an amenable group, R2, which therefore have a folner sequence, with the nth term something like Fn="all the points in the plane that are less than n from (0,0)" and then have the area of the overlap, O, be

limn to infty m(Fn intersect O)/m(Fn)

where m is normal lebesgue measure. This should be defined for all the sets we care about, and is translation invariant, which is another bonus.

That said, I expect the proof for the obvious tiling is going to be extremely non trivial.
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HenryS
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### Re: Circles on a plane

This would appear to be closely related to the circle packing problem, which according to http://mathworld.wolfram.com/CirclePacking.html, was solved in 1940 (which also means that someone has figured out how to define a density of packing on the plane that makes sense).

Perhaps a doable attack would be to try to reduce this circle covering problem to the circle packing problem?

Torn Apart By Dingos
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### Re: Circles on a plane

Owehn: What are absorbent sets?

jesting: I'm not able to follow that because I don't know what amenable groups or folner sequences are (amenable was on wikipedia but I couldn't find folner sequences). Was that some kind of proof that the way you measure the covering will not depend on what the sequence F_n are?

Owehn
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### Re: Circles on a plane

An absorbent subset A of a vector space over K is one with the property that for every element x of the vector space, there is a number k in K such that x/k is an element of A (that is, x is in kA). The idea is that as as you let k range over K, kA "absorbs" the whole vector space. Absorbent sets are also called absorbing.

An example of an absorbent set for R^2 is a disc around the origin.
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### Re: Circles on a plane

I had to register just to comment on this.

You never said all the circles had to be the same size. If they can get very small, you can cover the plane with a fractal-like pattern of circles so that each circle intersects only four others, and each point of the plane is contained in no more than two circles.

You can do this by tiling the plane with squares, then inscribing a circle in each square. The space outside the circles now looks like a bunch of regions each with four bounding arcs, and you can inscribe a circle in each of them. The remaining space now looks like a bunch of triangles (well "triangles" with arcs instead of straight sides), and you can inscribe a circle in each of those, leaving more triangles uncovered. Continue this process to inifinity and each point in the plane will eventually be engulfed, while each circle intersects no more than 4 others (each intersection is, in fact, a single point), and no 3 circles all intersect each other. The area of intersection also has measure zero (heck, it consists of a countable number of points). The final tiling looks sort of fractal-like (not unlike http://www.xkcd.com/17/). I think if you adjust the process a bit you can even make it so each circle only touches 3 others.

Okay, well, I'll go post in the intro thread now.

EDIT: Whoops, you did mention a few posts later that all the circles were of the same, finite size. Nevermind then.
Let's have a fervent argument, mostly over semantics, where we all claim the burden of proof is on the other side!

4=5
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### Re: Circles on a plane

draw a citcle around the fusilage
you have instant coverage with 2 circles

Torn Apart By Dingos
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### Re: Circles on a plane

Owehn wrote:An absorbent subset A of a vector space over K is one with the property that for every element x of the vector space, there is a number k in K such that x/k is an element of A (that is, x is in kA). The idea is that as as you let k range over K, kA "absorbs" the whole vector space. Absorbent sets are also called absorbing.

An example of an absorbent set for R^2 is a disc around the origin.
Wikipedia also required that x is in nA for all |n|>=k. Will that do what we want though?

The set {ax : |x|<1, a = angle(x), 0<a<=2pi} is absorbent in R^2 but can never be inflated to cover the unit disc. I'd rather use a set that covers some neighborhood of 0.

Owehn
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### Re: Circles on a plane

Definitions vary, so usually people use asborbing + convex, which will guarantee the property you want. In fact, I'll update my previous post with that condition.
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jestingrabbit
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### Re: Circles on a plane

Torn Apart By Dingos wrote:jesting: I'm not able to follow that because I don't know what amenable groups or folner sequences are (amenable was on wikipedia but I couldn't find folner sequences). Was that some kind of proof that the way you measure the covering will not depend on what the sequence F_n are?

The amenable groups article on WP isn't too bad (though not discussing folner sequences is an horrendous omission). It basically insists that there is a "mean" on the space of bounded functions,
where a mean, m, is such that m(1G)=1 (where 1G is the constant function on the amenable group G), m(sf + tg) = s(m(f)) + t(m(g)) (where f and g are real valued functions on G and s and t are real constants) and finally with m(f)>0 for f:G to R where f(x)>0 for all x in G. Think: m is something like an integral on LG.

So, I just say that R2 is an amenable group and demonstrate it by producing a folner sequence, which allows you to prove the existence of a mean m. In fact, if you take that formula from my earlier post, you will get the value of a mean on R2 for a subset of the plane, where you map that subset to its indicator function.

I don't know if those convex, absorbent sets give you the same deal, but I do know that the means that amenable groups give us exactly what we want. You could easily add the group of 2d rotations, the circle group, to the group of translations R2 and still have an amenable group, where we force the mean to behave well for rotations too. Either way, the idea of taking the mean of the sum of the indicator functions of the discs is what I want to go with. A formula for the mean of the sum of the indicators can be got at as follows, where I'll assume that the circles we use have radius 1. Let {(xi,yi)} be the set of all the centres of the circles, where i runs over the naturals. For r>0 let Nr = |{(xi,yi) : xi2+yi2<r2}| ie Nr is the number of circle centres inside the circle or radius r centred at the origin.

The mean of the sum of the indicator values of the discs is then

limr to ∞ r-2Nr

If you can tile the plane with some repeating segment, as you can with the assumed answer, then you just take the area of all the disc parts and divide by the area of the tile. I get a value of 2*π*3-3/2 = 1.20919958 for the assumed answer. Note that all of these means will be over 1, as 1 is less than the sum of the indicators at any point as the discs must cover all points.
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quintopia
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### Re: Circles on a plane

MartianInvader: If the sizes could vary, then we could cover the plane with no overlaps, and is thus a trivial problem. This is likely why it was formulated as it was.

HenryS
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### Re: Circles on a plane

quintopia wrote:MartianInvader: If the sizes could vary, then we could cover the plane
with no overlaps, and is thus a trivial problem. This is likely why it
was formulated as it was.
With the fractal covering plan? Wouldn't that leave out a Cantor dust type set of uncovered points?

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### Re: Circles on a plane

I'm not certain, but it seems to me that if you're talking about open circles, then the fractal-ish pattern would leave out points, but if you're talking about closed circles, then everything gets covered. Neither was specified at the start of the problem, so I was using closed circles.

quintopia: Yeah, I realized that the question was phrased in a non-trivial way after I posted, hence the edit at the end of my post.
Let's have a fervent argument, mostly over semantics, where we all claim the burden of proof is on the other side!

HenryS
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### Re: Circles on a plane

MartianInvader wrote:I'm not certain, but it seems to me that if you're talking about open circles, then the fractal-ish pattern would leave out points, but if you're talking about closed circles, then everything gets covered.
True. But yes, equal sized circles is a better problem.

Torn Apart By Dingos
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### Re: Circles on a plane

Okies.

MartianInvader wrote:I'm not certain, but it seems to me that if you're talking about open circles, then the fractal-ish pattern would leave out points, but if you're talking about closed circles, then everything gets covered.

Are you sure? With open circles, obviously the boundary will not be included, but will no other points be missed?

quintopia
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### Re: Circles on a plane

I'm pretty certain no points would be left out, but I'm not sure how to prove it.

phlip
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### Re: Circles on a plane

If you use solely open circles, then it'd be impossible to completely cover the plane without overlaps:
Spoiler:
Consider a point P on the border of circle A. Clearly, P is not covered by that circle. If P is covered by, say, circle B, then, because circle B is open, there exists an open disc centred on P that is contained entirely within circle B. However, because P is on the boundary of circle A, any open disc centred on P will intersect with circle A. Thus circle A and circle B must intersect.

This of course generalises to "You cannot cover the plane with a union of disjoint bounded open sets"... and I'm not even sure if "bounded" is necessary... maybe just requiring that the set has boundary points at all...

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quintopia
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### Re: Circles on a plane

Yes yes, but can you find a counter example to the closed set covering?

antonfire
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### Re: Circles on a plane

Yes.

Start with three congruent disks, pairwise mutually externally tangent, with one on the bottom, one to the top left, one to the top right, and consider the triangle-like region between them. You're removing disks from this region in the "fractal-like" way that was described above.

I'm going to assign to each sequence of the symbols 1, 2, and 3, a point in that region with all the open disks removed. 1 means go up, 2 means go down and right, 3 means go down and left. For example, a sequence starting with 1 represents some point in the top "third" of the region. A sequence starting with 22 represents some point in the bottom right "ninth". Then a point represented by a sequence is just the intersection (ranging over k) of the (closed) sets that it is limited to by the first k symbols in the sequence.

Now, this maps to a point on one of the circles if and only if the sequence eventually ends up only having two of the three symbols in it. So, an example of a point that's in the region with all the closed disks removed is the one represented by 123123123123... .

This is pretty much the exact same issue as with the Sierpinski gasket. The thing you get by drawing the lines isn't the same as the thing you get by removing the triangles.
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quintopia
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### Re: Circles on a plane

Does this counterexample extend to all fractal patterns? For example, what if the area between 3 circles was filled with a triangle of 3 in the opposite orientation?

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### Re: Circles on a plane

Kerberos wrote:
EdgarJPublius wrote:can you not just
Spoiler:
use one arbitrarily large circle?

Er... I forgot to mention that all the circles are of the same, finite size.

And Govalent, have you a proof?

...one large circle would be technically be the same size (as itself) and finite... maybe you should define the problem more accurately...
Let epsilon be less than zero...

quintopia
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### Re: Circles on a plane

1 finite circle cannot cover the plane, not matter how large. The problem is well-defined.

So what if you iterated the fractal for n steps and then threw just-big-enough disks around the uncovered regions? I don't see why this wouldn't result in the fraction of the plane covered by >1 disk being arbitrarily small. Yes, this breaks the "same size" rule, but it's still an interesting idea.

antonfire
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### Re: Circles on a plane

Yeah, it's pretty easy to see that that works. If you want to cover a region bounded by three circles, do the iterated fractal, and add e*2-n to the radius of the n'th disk (make it open). This adds a finite area to the disks (I'm too lazy to calculate what it is), and this area approaches zero as e goes to 0. So you can get arbitrarily good coverings. You can even take a finite subcover of this open cover of a compact region, so that you don't need arbitrarily small disks.

Next question: If you're allowed circles ranging in radius from 1 to R, how little overlap can you get?
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