Proof: Integral is finite (Fubini/Tonelli?)

In summary, the problem given requires proving the finiteness of the integral \int_{[0,1]}\frac{f(y)}{|x-y|^{1/2}}dy for almost every x in [0,1], given that f:[0,1]→ℝ is non-negative and integrable. The solution involves using Fubini/Tonelli's Theorem and considering the integrability of the function f(y) = |x-y|^{-\frac{1}{2}} for y≠x. By using Tonelli's Theorem and some additional qualifiers, it can be shown that the integral is indeed finite for almost every x in [0,1].
  • #1
ChemEng1
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Homework Statement


Let f:[0,1]→ℝ be non-negative and integrable. Prove that [itex]\int_{[0,1]}\frac{f(y)}{|x-y|^{1/2}}dy[/itex] is finite for ae x in [0,1]

Homework Equations


This looks like a Fubini/Tonelli's Theorem problem from the problem givens.

The Attempt at a Solution


I honestly don't know where to start with this problem. Any help or gentle nudges would be appreciated.
 
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  • #2
It doesn't seem to me to be true, so I must be missing something.
Given x in (0, 1), consider ##f(y) = |x-y|^{-\frac{1}{2}}## for y≠x, f(x)=0. Isn't that integrable? But ##\int_0^1|x-y|^{-1}dy## is not finite.
 
  • #3
Here's what I am thinking.

Consider: [itex]\int_{[0,1]}f(y)\left[\int_{[0,1]}\frac{1}{|x-y|^{1/2}}dx\right]dy=\int_{[0,1]}f(y)\left[2\left(\sqrt{1-y}-\sqrt{y}\right)\right]dy\leq\int_{[0,1]}f(y)\cdot 2<∞[/itex]. Therefore [itex]\int_{[0,1]^{2}}\frac{f(y)}{|x-y|^{1/2}}<\infty[/itex] by Tonelli's Theorem. Then [itex]\int_{[0,1]}\frac{f(y)}{|x-y|^{1/2}}dy[/itex] is finite for ae x in [0,1] again by Tonelli.

Think I still need to add some measurability and non-negative qualifiers, but this is close.

Thoughts?
 
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1. What is the proof for the finiteness of an integral using Fubini/Tonelli?

The proof for the finiteness of an integral using Fubini/Tonelli relies on the properties of measure theory and the fact that the integral is defined as the limit of a Riemann sum. By using the monotone convergence theorem and the dominated convergence theorem, it can be shown that the integral is finite under certain conditions.

2. What are the conditions under which the integral is finite using Fubini/Tonelli?

The integral is finite using Fubini/Tonelli if the function being integrated is measurable and either non-negative or absolutely integrable. Additionally, the domain of integration must be a measure space, and the function must satisfy the conditions of the monotone convergence theorem and the dominated convergence theorem.

3. How does Fubini's theorem contribute to the proof of the finiteness of an integral?

Fubini's theorem allows for the interchange of the order of integration in a double integral, which is useful in proving the finiteness of an integral. By rearranging the order of integration and using the properties of measure theory, Fubini's theorem can help to simplify the integral and make it easier to evaluate for finiteness.

4. What is the role of Tonelli's theorem in the proof of the finiteness of an integral?

Tonelli's theorem is a generalization of Fubini's theorem, which allows for the interchange of the order of integration in a multiple integral with respect to any measure. This is useful in proving the finiteness of an integral, as it allows for the integration to be rearranged and evaluated in a way that may be easier to determine finiteness.

5. Can the finiteness of an integral be proven without using Fubini/Tonelli?

Yes, the finiteness of an integral can be proven using other methods, such as the Lebesgue integral. However, the use of Fubini/Tonelli theorems can often simplify the proof and make it more intuitive. Additionally, Fubini/Tonelli theorems are necessary for evaluating certain types of integrals, such as multiple integrals, which may not be possible using other methods.

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