# How to prove using cartesian product

• November 13th 2009, 09:04 AM
brudman
How to prove using cartesian product
How can one prove that for all sets A and B, if A and B are nonempty and A X B = B x A, then A = B.
• November 13th 2009, 09:15 AM
Plato
Quote:

Originally Posted by brudman
How can one prove that for all sets A and B, if A and B are nonempty and A X B = B x A, then A = B.

Start with $(x,y)\in A\times B$ implies that $x\in A~\&~y\in B$.
If $A\times B=B\times A$ what can be said of $x~\&~y?$
• November 15th 2009, 09:25 AM
brudman
x = y ??
• November 15th 2009, 02:47 PM
emakarov
Quote:

x = y ??
No, x and y are even from different sets (like apples and oranges).

OK, so you need to prove that A = B. It is sufficient to show that every element of A is an element of B, and conversely. Let's prove the first part.

What is the standard beginning of a proof of a statement of the following form: "Every such-and-such object has such-and-such property"? It's "Let's fix any such-and-such object". Write in symbols what this means here.

Next, B is nonempty, so choose some element. Now we have one element from A, one from B. What can we form from them?

OK, now A x B = B x A, so this object we formed is also in B x A. Now use Plato's remark. This should conclude the proof that A is a subset of B. The opposite inclusion is similar.