## Archive for the ‘Fractals’ Category

### A decagonal snowflake from pentagons

25 June 2017

So far in these posts about fractals, I’ve shown (1) how letting triangles reproduce like this:

generates a bunch of copes of the Koch snowflake at different scales:

Similarly, I’ve shown (2) how letting squares reproduce like this:

generates a bunch of copies of a fractal related to the Koch snowflake, but with 8-fold symmetry:

So what about letting pentagons reproduce?   For pentagons, an analog of the replication rules above is this:

Each of the 10 cute little pentagon children here is a factor of $\frac{1}{2+\varphi}$ smaller than its parent, where $\varphi$ is the golden ratio.

However, something interesting happens here that didn’t happen with the triangle and square rules. While triangles and squares overlap with their ancestors, pentagons overlap with both their ancestors and their cousins.  The trouble is that certain siblings already share faces (I know, the accidental metaphors here are getting troublesome too!), and so siblings’ children have to fight over territory:

In this three-generation pentagon family portrait, you can see that each second generation pentagon has two children that overlap with a cousin.

As we carry this process further, we get additional collisions between second cousins, third cousins, and so on.  At five generations of pentagons, we start seeing some interestingly complex behavior develop from these collisions:

There’s a lot of fascinating structure here, and much of it is directly analogous to the 6-fold and 8-fold cases above, but there are also some differences, stemming from the “cousin rivalry” that goes on in pentagon society.

Let’s zoom in to see some collisions near where the two ‘wreaths’ meet on the right side of the picture:

I find the complicated behavior at the collisions quite pretty, but the ordering issues (i.e. which members of a given generation to draw first when they overlap) annoy me somewhat, since they break the otherwise perfect decagonal symmetry of the picture.

If I were doing this for purely artistic purposes, I’d try resolving the drawing order issues to restore as much symmetry as possible. Of course, I could also cheat and restore symmetry completely by not filling in the pentagons, so that you can’t tell which ones I drew first:

It’s cool seeing all the layers at once in this way, and it shows just how complex the overlaps can start getting after a few generations.

Anyway, because of these collisions, we don’t get seem to get a fractal tiling of the plane—at least, not like we got in the previous cases, where the plane simply keeps getting further subdivided into regions that converge to tiles of the same shape at different scales.

Actually, though, we still might get a fractal tiling of the plane, if the total area of overlap of nth generation pentagons shrinks to zero as n goes to infinity!  That would be cool.  But, I don’t know yet.

In any case, the picture generated by pentagons is in many ways very similar to the pictures generated by triangles and squares. Most importantly, all of the similar-looking octagonal flower shaped regions we see in this picture including the outer perimeter, the inner light-blue region, and tons of smaller ones:

really are converging to the same shape,  my proposal for the 10-fold rotationally symmetric analog of the Koch snowflake:

How do we know that all of these shapes are converging to the same fractal, up to rescaling?  We can get a nice visual proof by starting with two pentagons, one rotated and scaled down from the other, and then setting our replication algorithm loose on both of them:

Proof:

We see that the area between the two fractal curves in the middle shrinks closer to zero with each generation.

Puzzle for Golden Ratio Fans: What is the exact value of the scaling factor relating the two initial pentagons?

Next up in this infinite series of articles: hexagons!  …

I’m joking!  But, it’s fairly clear we can keep ascending this ladder to get analogs of the Koch snowflake generated by n-gons, with (2n)-fold rotational symmetry.  More nice features might be sacrificed as we go up; in the case generated by hexagons, we’d have collisions not only between cousins, but already between siblings.

### More fractal fun

23 June 2017

In the previous article, I explained how the famous Koch snowflake can be built in a different way, using “self-replicating” triangles. This was a revelation for me, because I had always thought of the Koch snowflake as fundamentally different from other kinds of fractals like the Sierpinski triangle, and now I think of them as being basically the same.

In the Sierpinski triangle, each triangle yields three new, scaled-down triangles, attached at the midpoints of sides of the original, like this:

These triangles are usually thought of as “holes” cut out of a big triangle, but all I care about here is the pattern of the triangles.  As I explained last time, the Koch snowflake can be built in a similar way, where each triangle produces six new ones, like this:

You might say this bends the usual rules for making fractals since some of the triangles overlap with their ancestors.  But, it makes me happy because it lets me think of the Sierpinski triangle and the Koch snowflake as essentially the same kind of thing, just with different self-replication rules.

What other fractals can we build in this way?  The Sierpinski carpet is very similar to the Sierpinski triangle, where we now start with squares and a rule for how a square produces 8 smaller ones:

This made me wonder if I could generalize my construction of the Koch snowflake using triangles to generate some other fractal using squares.  In other words, is there some Koch snowflake-like fractal that is analogous to the ordinary Koch snowflake in the same way that the Sierpinski carpet is analogous to the Sierpinki traingle?

There is!  Taken to the 5th generation, it looks like this:

The outline of this fractal is an analog of the Koch snowflake, but with 8-fold symmetry, rather than 6-fold.  Compare the original Koch snowflake with this one:

Just as I explained last time for the Koch snowflake (left), the blue picture above actually provides a proof that the plane can be tiled with copies of tiles like the one on the right, with various sizes—though in this case, you can’t do it with just two sizes of tiles; it takes infinitely many different sizes!  In fact, this tiling of the plane is also given in Aidan Burns’ paper I referenced in the previous post.

But, my construction is built entirely out of self-replicating squares.  What’s the rule for how squares replicate?

Before I tell you, I’ll give two hints:

First, each square produces 8 new squares, just like in the Sierpinski carpet.  (So, we could actually make a cool animation of this fractal morphing into the Sierpinski carpet!)

Second, you can more easily see some of the bigger squares in the picture if you make the colors of the layers more distinct.  While I like the subtle effect of making each successive generation a slightly darker shade of blue, playing with the color scheme on this picture is fun.  And I learned something interesting when my 7-year old (who is more fond of bold color statements) designed this scheme:

The colors here are not all chosen independently; the only choice is the color of each generation of squares.  And this lets us see bits of earlier-generation squares peeking through in places I hadn’t noticed with my more conservative color choices.

For example, surrounding the big light blue flower in the middle, there are 8 small light blue flowers, and 16 even smaller ones (which just look like octagons here, since I’ve only gone to the 5th generation); these are really all part of the same light-blue square that’s hiding behind everything else.

The same thing happens with the pink squares, and so on.  If you stare at this picture, you can start seeing the outlines of the squares.

So what’s the rule?  Here it is:

### Fun with the Koch snowflake

22 June 2017

I was playing with fractals recently and discovered a fun way to construct the Koch snowflake. It may not be new to the world, given how much is known about fractals, but it was new to me. In any case, it’s really cool!  It lets you see a lot more interesting self-similarity in this fractal, and effortlessly leads to the known tilings of the plane by Koch snowflakes.

The  Koch snowflake is usually constructed starting with an equilateral triangle by replacing the middle third of each side with an equilateral triangular protrusion, doing this again to the resulting polygon, and so on.  The first seven steps are shown in this animation:

and the Koch snowflake is the “limit” of this process as the number of steps goes to infinity.

In the alternative construction we use only self-replicating triangles.  We again start with a triangle:

But now, rather than modifying this triangle, we let it “reproduce,” yielding  six new triangles, three at the corners of the original, and three sticking out from the sides of the original.  I’ll make the six offspring a bit darker than the original so that you can see them all:

Notice that three of the children hide the corners of their parent triangle, so it looks like we now have a hexagon in the middle, but really we’ve got one big triangle behind six smaller ones.  Now we do the same thing again, letting each of the six smaller triangles reproduce in the same way:

The 36 small triangles are the “grandchildren” of the original triangle; if each of these has six children of its own, we get:

Repeating this again:

And again:

At this stage, it starts getting hard to see the new triangles, so I’ll stop and rely on your imagination of this process continuing indefinitely.  We can now see some interesting features emerging.  Here are some of the main ones:

1. The outer perimeter of all of these triangles, taken to the infinite limit, is the Koch snowflake.
2. The lightest blue region, in the middle, is also converging to a smaller Koch snowflake, rotated from the outer one by $\pi/6$.
3. Between the outer perimeter and the Koch snowflake in the middle are six more yet smaller Koch snowflakes.
4. The regions in the middle of these snowflakes are also Koch snowflakes …

and so on: we have Koch snowflakes repeating at smaller and smaller scales.

All this self similarity shows in particular that Koch snowflakes can be assembled out of Koch snowflakes.  This is nothing new; it’s related to Aidan Burns’ nice demonstration that Koch snowflakes can be used to tile the plane, but only if we use snowflakes of at least two different sizes:

Aidan Burns, Fractal tilings. Mathematical Gazette 78 No. 482 (1994) 193–196.

These tilings are already visible in the above construction using triangles, but we can make them even more evident by just playing with the color scheme.

First, if we draw the previous picture again but make all of the triangles the same color, we just get a region whose perimeter is the usual Koch snowflake:

On the other hand, if we make the original triangle white, but all of its descendants the same color of blue, we get this:

I hope you see how this forms part of a wallpaper pattern that could be extended in all directions, adding more blue snowflakes that bound larger white snowflake-shaped gaps. This gives the tiling of the plane by Koch snowflakes of two different sizes.

Taking this further, if we make the the original triangle and all of its children white, but all of their further descendants the same color of blue, we get this:

The pattern seen here can be extended in a hopefully obvious way to tile the whole plane with Koch snowflakes of three different sizes.

Going further, if we make the first three generations white, but all blue after that, we get:

and so on.

The previous four pictures are generated with exactly the same code—we’re drawing exactly the same triangles, and only changing the color scheme.  If we keep repeating this process, we get a tiling with arbitrarily small Koch snowflakes!

But we can also go the other way, continuing up in scale to get a tiling that includes arbitrarily large Koch snowflakes.  To do this, we just need to view the above series of pictures in a different way!

The way I described it, the scale is the same in all of the previous four pictures.   Making successive generations white, one generation at a time, makes it look as if we’re cutting out a snowflake from the middle of a big snowflake, leaving six similar snowflakes behind, and then iterating this:

On the other hand, we can alternatively view these pictures as a process of zooming out: each picture is built from six copies of the previous one, and we can imagine zooming out so that each picture becomes just one small piece of the next:

etc.

If you’re careful about how you do this, you get a tiling of the whole plane, with arbitrarily large Koch-snowflake shaped tiles!  I say you have to be careful because it won’t cover the whole plane if, for example, each picture becomes the top middle part of the next picture.  But, if you zoom out in a “spiral,” rotating by $\frac{\pi}{3}$ at each step, you’ll get a tessellation of the plane.

Someone should make an animation that shows how this works.  Maybe I’ll get a student to do it.

There are some other fun variations on this theme—including a similar construction that leads to the other “fractal tiling” described by Aidan Burns—which I should explain in another post.

In case anyone wants it, here is a 1-page visual explanation of the construction described in this post: snowflake.pdf

(All images in this post, except for the first, copyright 2017 Derek Wise.)