ELI5: Explain Like I'm 5

Ending lamination theorem

Imagine that you have a piece of paper and you fold it in half. Now imagine you fold it in half again, and keep doing that until you have a piece of paper that is really small. This is kind of like what happens in mathematics when we talk about a "sequence" of things getting smaller and smaller.

The Ending Lamination Theorem is a special rule that helps us understand what happens to a sequence of shapes as they become really small. Specifically, it helps us understand what happens when we're looking at the boundaries (or edges) of those shapes.

To understand this theorem, we need to know a few things. First, we need to know that some shapes (like circles and squares) have edges that are "smooth." This means that if you zoom in really close to the edge of the shape, it looks like a straight line.

Other shapes (like triangles and polygons) have edges that are "rough." This means that if you zoom in really close to the edge of the shape, it looks bumpy and broken up.

The Ending Lamination Theorem tells us that if we have a sequence of shapes, and we look at the boundaries of those shapes as they get smaller and smaller, they will eventually "collapse" onto a smooth line. This means that even if the edges of the individual shapes were bumpy or broken, when we look at them all together, they will be smooth.

So, imagine if you take a big square and divide it into four smaller squares. Then divide those into even smaller squares, and keep going until you have squares that are too small to see. If you look at the edges of all those squares together, they will eventually become smooth and form a smooth line.

This is important because it helps us understand how some shapes can be built up from simpler shapes. If we know that the edges of all those simpler shapes will eventually be smooth, then we can understand how the edges of more complex shapes will also be smooth.