Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Show that the Lebesgue measurable sets $A,B$ satisfy $|A \times B| = |A||B|$. The approach I was going to use, was to use Tonelli's theorem which says $$ \int \limits_{A \times B}f = \int \limits_{A} \left[ \int \limits_{B}f(x,y) dy \right] dx $$ Then use a theorem that says $$ \int \limits_{A}f = \sum \limits_{i=1}^m a_j |A_{j}| $$ This will get me $$ \sum \limits_{i=1}^m a_j |A_{j} \times B_{j}| = \sum \limits_{j=1}^m \left( \sum \limits_{i=1}^n b_i |B_{i}| \right) _j |A_{j}| $$ but I am not sure how I could manipulate this to remove the summation. The only method I could think of would be to say $f$ is a constant for a certain interval such that $$ \int \limits_{A}f = a |A| $$ then, when $a=b$ $$ a |A\times B| = b |B||A| \Rightarrow |A\times B| = |B||A| $$ Is that a stretch?

Edit: $$ A \times B = \lbrace (x,y)\in A \times B : x\in A , y\in B \rbrace $$

share|cite|improve this question
How are you defining $|A \times B|$? – Christopher A. Wong Oct 31 '12 at 0:12
I never understand how it can seem appealing to write "thm" and "const" instead of "theorem" and "constant" when it saves you three or four keystrokes each while forcing several dozen prospective readers to break their reading flow and think about the intended meaning of the abbreviation -- especially when the people at whose expense you're skimping on keystrokes are the ones you're asking for help. – joriki Oct 31 '12 at 0:26
Is that better:)? – rioneye Oct 31 '12 at 0:28
Yes, thanks :-) – joriki Oct 31 '12 at 0:30
Maybe you know that $|A\times B|=|A|\cdot |B|$ if $A$ and $B$ are Borel measurable. Do you see why this will be true for _Lebesgue measurable sets? – Davide Giraudo Oct 31 '12 at 10:43

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.