# Homework Help: Simple Set Theory Question

1. Jan 1, 2006

### Ed Quanta

If E has m elements and F has n elements, how many elements does E x F have?

My thinking is that E x F would either have m or n elements. If m= n, then E x F would have m elements (or n elements). If m>n, then E x F would have n elements since E x F ={(x,y): x is an element of E and y is an element of F}. This of E has more elements than F, then there could only be n (x,y) pairs since there would only be n y's.Am I correct?

Also, if E x F is an empty set, then it would follow that E or F would have to be an empty set,right?

2. Jan 1, 2006

### fourier jr

think of it as finite sets of points in the plane. the set MxN would be a "rectangle" with mn points. it's true even if M & N are infinite but that's a bit more complicated.

3. Jan 2, 2006

### matt grime

Did it not occur to try a couple of examples, say with small sets with a couple of elements in each?

4. Jan 2, 2006

### Ed Quanta

Thanks Fourier

5. Jan 4, 2006

### Ed Quanta

Can someone provide me with an example where the cartesian product
E x F is a subset of G x H but it does not follow that E is a subset of G and F is a subset of H?
I have been trying to come up with an example that satisifies this condition for the last day or so.
I can see why it would be true that E is a subset of G and F is a subset of H would imply E x F is a subset of G x H. But I am not sure about the reverse direction.

6. Jan 4, 2006

### Muzza

Try letting E or F be the empty set.

7. Jan 4, 2006

### mathwonk

Actually the answer to your question is tautological, i.e. mn is by definition the number of elements of a set MxN where m = card(M) and n = card(N).

In other words the set MxN is considered by many as more basic than the notion of multiplication.

This not so stupid as it appears since the product set makes sense for infinite sets whereas the product of cardinal numbers does not, a priori.

But one even more basic thing has puzzled me all these years: what is a set?:tongue2:

Last edited: Jan 4, 2006
8. Jan 5, 2006

### fourier jr

suppes says it's something that either has elements in it, or is the empty set

9. Jan 5, 2006

### HallsofIvy

So a set is something that has elements- or doesn't???:yuck:

I'm moving this to the set theory forum.

10. Jan 5, 2006

### fourier jr

lol he says "y is a set <--> there exists x such that x is an element of y or y is the empty set" (p.19)