I’ve written a couple blog posts that may seem to contradict each other. Given a high-dimensional cube, is most of the volume in the corners or not?
I recently wrote that the corners of a cube stick out more in high dimensions. You can quantify this by centering a ball at a corner and looking at how much of the ball comes from the cube and how much from surrounding space. That post showed that the proportion of volume near a corner goes down rapidly as dimension increases.
About a year ago I wrote a blog post about how formal methods let you explore corner cases. Along the way I said that most cases are corner cases, i.e. most of the volume is in the corners.
Both posts are correct, but they use the term “corner” differently. That is because there are two ideas of “corner” that are the same in low dimensions but diverge in higher dimensions.
Draw a circle and then draw a square just big enough to contain it. You could say that the area in the middle is the area inside the circle and the corners are everything else. Or you could say that the corners are the regions near a vertex of the square, and the middle is everything else. These two criteria aren’t that different. But in high dimensions they’re vastly different.
The post about pointy corners looked at the proportion of volume near the vertices of the cube. The post about formal methods looked at the proportion of volume not contained in a ball in the middle of the cube. As dimension increases, the former goes to zero and the latter goes to one.
In other words, in high dimensions most of the volume is neither near a vertex nor in a ball in the middle. This gives a hint at why sphere packing is interesting in high dimensions. The next post looks at how the sphere packings implicit in this post compare to the best possible packings.