ELI5: Explain Like I'm 5

Joint spectral radius

Have you ever played with building blocks? Imagine you have a set of different types of blocks, like the flat ones, the square ones, and the triangular ones. Now, let's say you want to stack them up neatly and see how high you can go.

In a similar way, when people want to solve problems with a lot of steps, they often break it up into smaller problems that can be solved one at a time, and then put together those solutions to solve the bigger problem. This is called "decomposition."

Sometimes, these smaller problems involve something called a matrix - which is a grid of numbers or symbols. If you have two or more matrices, you can do something called a "product," which is like stacking the blocks.

The joint spectral radius is a way to measure how high you can stack these products. It's like asking, "if I stack these matrices up, how much will they grow in size?"

Let's say you have a matrix representing a problem, and it's not really easy to solve. But then, you can "decompose" that matrix into two or more smaller matrices that are easier to solve. You can then stack those smaller matrices (like putting blocks together), and the joint spectral radius tells you how much bigger the stacked matrices will be compared to the original matrix.

So, in summary: the joint spectral radius is a measure of how much matrices grow in size when you stack them together, which is often useful for solving complex problems by breaking them down into smaller pieces.