ELI5: Explain Like I'm 5

Ramsey cardinal

Okay, imagine you have a big pile of toys that you love to play with. Now, you want to count how many toys you have, but you don't want to count each toy one by one because it will take forever. So, you decide to put all your toys into different groups, and you count the number of groups you have. For example, you may put all your stuffed animals in one group, all your cars in another group, and so on.

Now, let's talk about the Ramsey cardinal. This is a very big number that tells you how many groups you need to make, so that no matter how you divide your collection of toys, one of the groups will always have a specific property. This property is called being "homogeneous" and it means that all the toys in that group have something in common.

For example, let's say you have 10 toys and you want to divide them into groups, so that one group will have only red toys and the other group will have only blue toys. You can make two groups and put all the red toys in one group and all the blue toys in the other group. Now, no matter how you shuffle the toys around, one of the groups will always have only red toys, and the other group will always have only blue toys. So, the Ramsey cardinal in this case is 2.

Now, imagine you have a really big set of toys, like millions and millions of them. The Ramsey cardinal for this set is a very, very big number. In fact, it's so big that we can't even write it down using regular numbers. It's like trying to count all the grains of sand on a beach!

So, to sum it up, the Ramsey cardinal is a number that tells you how many groups you need to make, so that one of the groups will always have a special property, no matter how you divide your collection. And for very big sets, the Ramsey cardinal is a mind-bogglingly huge number!