1. Feb 21, 2005

### Townsend

I have been through elementary set theory but there are still a few things that I never really understood in that class. One that keeps coming up and I still cannot resolve is this.

If two sets A and B are disjoint sets then neither is a subset of the other unless one is the empty set.

Looking at the definition of disjoint we have:

Two sets A and B are disjoint if they have no elements in common.

And we also have that the empty set is the subset of every set.

So what if we let A be the empty set and B be the empty set?

Are they disjoint then?

Or consider A={1,2,3,{}} and B={}

Now A and B are disjoint and B is a subset of A. What does that mean? They both contain the empty set, and that is a common element for every set, right. I mean in order for the empty set to be a subset of every set then every set must contain the empty set.

I think I don't really understand the concept of what an empty set is. I realize that the set with no element is the empty set but the set itself is an element, no? If I give you three sets with definitions for all the elements in them then the three sets can also be considered elements of another set right? So if one of those sets is defined to be the null set then it is still an element of another set.

So now we look at sets A and B above and we see they both contain the empty set. So they are not really disjoint and no two sets can be disjoint if they all have a common element namely the element that is the empty set.

I know that I have this all wrong but I really need to understand this concept. If someone from PF can make this as simple as possible I would really appreciate it a lot.

Thanks

2. Feb 21, 2005

### matt grime

yes, AnB is empty.

B does not contain the empty set, it is the empty set. B contains no elements. The empty set is a subset of every set, not an element of every set. You are confusing the two things.

No, the empty set is not an element of itself.