Proof: Integral is finite (Fubini/Tonelli?)

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SUMMARY

The integral \(\int_{[0,1]}\frac{f(y)}{|x-y|^{1/2}}dy\) is finite for almost every \(x\) in the interval \([0,1]\) when \(f:[0,1] \to \mathbb{R}\) is non-negative and integrable. This conclusion is established using Tonelli's Theorem, which allows the interchange of integration and ensures the finiteness of the double integral \(\int_{[0,1]^{2}}\frac{f(y)}{|x-y|^{1/2}}<\infty\). The proof requires careful consideration of the measurability and non-negativity of the function \(f\).

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  • Understanding of Fubini's and Tonelli's Theorems
  • Knowledge of Lebesgue integration
  • Familiarity with non-negative functions and their properties
  • Basic concepts of measure theory
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Homework Statement


Let f:[0,1]→ℝ be non-negative and integrable. Prove that \int_{[0,1]}\frac{f(y)}{|x-y|^{1/2}}dy 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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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.
 
Here's what I am thinking.

Consider: \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&lt;∞. Therefore \int_{[0,1]^{2}}\frac{f(y)}{|x-y|^{1/2}}&lt;\infty by Tonelli's Theorem. Then \int_{[0,1]}\frac{f(y)}{|x-y|^{1/2}}dy 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?
 
Last edited:

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