ELI5: Explain Like I'm 5

Representation theory of SL2(R)

Okay, kiddo! Do you know what a toy car is? It's a miniature version of a real car that you can play with. Similarly, mathematicians have a toy version of a group called SL2(R), which stands for Special Linear group of degree 2 over real numbers. This group is composed of matrices with 2 rows and 2 columns, and certain rules that they must follow when added, subtracted, and multiplied together.

Now, imagine that we want to understand how this toy group works when we take it very seriously, like grown-up mathematicians. One way to do that is to study something called its representation theory. Representation theory is all about figuring out how the members of a group act on different objects, such as vectors. In other words, we want to see how this toy car behaves when we play with it in different ways!

For example, imagine that we have a toy car that can move from one spot to another on a toy road. We may want to know how it moves when we push it backward, or when we turn it in certain directions. Similarly, when it comes to SL2(R), we want to know how each of its matrices affects vectors, which are like little arrows in space with certain directions and lengths.

To study this, we use a special language called linear algebra, which is like a secret code that only mathematicians can decipher. We write down equations that describe how the matrices in SL2(R) act on vectors, and we look for patterns in these equations. Some of these patterns are very useful, and they help us understand the toy group better.

For instance, we might discover that some of the matrices in SL2(R) can be written as a combination of other matrices that we already know well. This means that we can build complex actions out of simpler ones, like building a big toy castle with smaller toy bricks.

Overall, representation theory of SL2(R) is a way for mathematicians to study the behaviors of a toy group made up of matrices, and how they interact with vectors. By figuring out the rules and patterns behind these interactions, we can learn more about how this toy group works, and maybe even find new applications for it in the real world. Pretty neat, huh?