ELI5: Explain Like I'm 5

Chebychev–Grübler–Kutzbach criterion

Okay kiddo, the Chebychev-Grübler-Kutzbach (CGK) criterion is a rule that helps us figure out how many ways something can move. Imagine you have a toy car with wheels. How many directions can your car move? It can go forward, backward, left, right, and turn. That’s 5 different ways it can move. But what if you had a toy train with lots of wheels and joints? The CGK criterion tells you how many possible ways it can move!

It works like this. First, you count how many parts your train has that can move. This can be wheels or joints or anything that moves. Let’s say it has 10. Then, you use a fancy formula to figure out how many ways it can move. The formula looks like this:

W = 3(N-1) - 2J

Don't worry, we will break it down. Here, W is the number of ways the train can move. N is the number of parts that can move, which is 10 in our example. J is the number of joints or flexible parts, like hinges or ball-and-socket joints.

So, if your train has no joints, J=0, and you just plug in the numbers:

W = 3(10-1) - 2(0)
W = 27

That means your train can move in 27 different ways! If your train has 5 joints instead, J=5, and we use the same formula:

W = 3(10-1) - 2(5)
W = 17

So, with 5 joints, the train can move in 17 different ways. That's still pretty good!

The CGK criterion is important in engineering and robotics because it helps us design machines and robots that can move in lots of different ways. By knowing how many ways something can move, engineers can figure out how to make it move more efficiently or do specific tasks. Isn't that neat?