by Jonathan Kujawa
Apollonius of Perga (262-190 BC) was a well known and prolific geometer in ancient Greece. He is mainly known for his surviving work on the conic sections. Indeed, he gave us the definition of the ellipse, parabola, and hyperbola we use today. In some circles, Apollonius's most famous theorem is the fact that if you have three circles which are mutually tangent, then there always exactly two ways to add a fourth circle which is tangent to all three. That is, you can always fit exactly one new circle tangent to the original three within each interstice made by the existing circles. In the picture to the right if you start with the three black circles, then you can complete the picture to four tangent circles by adding either of the gray circles.
One thing I've learned in math is: anything worth doing once, is worth doing many times. Once you add the two new circles you've now created six new gaps. This, in turn, can be filled with circles, creating yet more gaps. And so on. You can keep adding circles forever:
The fractally looking result is an Apollonian Gasket. There is a delightful Gasket maker available here. You get fun pictures like this one:
As we did above, it is common when drawing Gaskets to start with two circles inside of a third, but this is only for convenience and isn't needed for Apollonius's result. Indeed, if you start with three circles which are tangent to each other and none contains the others, then one of the two new circles you add will encircle the others, so you'll end up with the outside circle, anyway.
When looking at pictures like these, it is natural to ask how much of the big circle is filled by the smaller circles. At each stage there will always be little slivers of empty space. But perhaps eventually every gap is filled with a circle. If you know about fractals, you'll know the answer to this question is not so obvious.