Okay kiddo, let's talk about the Plancherel theorem for spherical functions!
Imagine you have a big ball, like a basketball. If you shine a light on it, the light will bounce off the ball in different directions. The way the light bounces off the ball is kind of like how sound waves bounce off different surfaces and create echoes.
But instead of light or sound waves, we're talking about functions. A function is kind of like a set of instructions that tells you what to do with a number. For example, if you have a function that multiplies a number by 2, when you put in the number 5, the function would give you the answer 10.
Now, let's say we have a function that works with the ball we talked about earlier. This function is called a spherical function, because it has to do with the way the ball is shaped like a sphere. We can think of the function as giving us information about how the ball looks or behaves.
The Plancherel theorem helps us understand how spherical functions work. It says that if we take two different spherical functions and multiply them together, then add up all the answers we get from doing that for every possible combination of numbers, the result will be the same as if we took each individual function and squared it, then added up all those answers.
This might sound a little complicated, but basically it means that when we use spherical functions to describe things like the way sound or light bounces off a sphere, we can break down the information we get into smaller pieces and understand it better. It's like taking apart a puzzle and seeing how each piece fits together.
In summary, the Plancherel theorem helps us understand how spherical functions work by showing us how we can break down information about a sphere into smaller pieces and put them back together again.