# Homework Help: Integral of product spaces

1. Dec 2, 2012

### happysauce

1. The problem statement, all variables and given/known data

I just have a question about the integral of a product space. How do I define the integral of product spaces in terms of characteristic functions?

What I mean by that is, if I have a measure space, (X,M,u) and f(x) is a positive, simple, measurable function. Then ∫f du = Ʃa$_{i}$u(E$_{i}$). What I want to know is how can I apply this to a simple function given the product space of (X,M,u) and (Y,N,v)?

The problem I have to do is to prove that ∫f(x)g(y)d(u×v)=(∫f(x)du)(∫g(y)dv), you can't use fubini's theorem since the problem doesn't assume the measure spaces are sigma finite and the hint suggested using standard limit theorems for integrals, which made me think I probably had to use simple functions...

2. Dec 2, 2012

### gopher_p

The same as any other measure space.

The same as any other measure space.

This sounds like a good strategy. But you probably want to restrict yourself to simple functions of the form $f(x)=\sum a_i\chi_{E_i}(x)$ where the $E_i$ are "rectangles" in the product space. Can you decompose the characteristic function of a rectangular region into a product of characteristic functions on the "component" spaces?

3. Dec 3, 2012

### happysauce

That's what I tried. I took the product of the integrals and expressed them as a product of two sums [$\sum$a$_{i}$u(E$_{i}$) ][$\sum$b$_{j}$v(E$_{j}$)], one summed n parts the other summed m parts. Then I noticed that the product created a sum of n×m rectangles and I was able to express it in terms of one sum.

4. Dec 3, 2012

### micromass

OK, so you proved the theorem for the special case simple functions, where for every $\chi_{E_j}$ holds that $E_j$ is a rectangle. What did you do next?