Is the Given Statement a Consequence of Fubini's Theorem?

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Discussion Overview

The discussion revolves around the implications of Fubini's theorem in relation to the properties of functions in the context of Lebesgue spaces. Specifically, it examines whether the condition of a function \( f \) being in \( L^p \) space leads to the conclusion that its partial derivatives exist in \( L^p \) space for almost all points in the domain.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the statement about the function \( f \) being in \( L^p \) implies that its partial derivatives \( f_y(x) \) and \( f_x(y) \) are also in \( L^p \) for almost all \( y \) and \( x \), respectively.
  • Another participant asserts that the statement is true, suggesting that if \( g \) is in \( L^p \), then \( |g|^p \) is in \( L^1 \), and proposes applying Fubini's theorem to \( |f(x,y)|^p \).
  • A third participant provides feedback on the formatting of mathematical expressions, indicating a technical issue unrelated to the mathematical content.

Areas of Agreement / Disagreement

There is a disagreement regarding the correctness of the initial claim about the implications of Fubini's theorem, with one participant affirming the claim while another has not yet responded to challenge or support it further.

Contextual Notes

The discussion does not clarify the assumptions under which the claims are made, nor does it resolve the implications of the conditions stated in the original question.

Who May Find This Useful

Participants interested in functional analysis, particularly those studying properties of functions in Lebesgue spaces and the applications of Fubini's theorem.

Tatianaoo
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Does anybody know if the following is true?

Let $p>1$. If $f=f(x,y)$ is such that $f\in L^p([a,b]\times [a,b])$, then $f_y(x)\in L^p([a,b])$ for almost all $y\in[a,b]$ and $f_x(y)\in L^p([a,b])$ for almost all $x\in[a,b]$.

Is this a consequence of Fubini's theorem?
 
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Unfortunately, $ don't work. You need to enclose your code with [/itex] and . The one with the slash goes at the end but I had to put them in reverse order for it to show up (or it would have just treated "and" as my code.
 
The answer to your question is yes. g is Lp if and only if |g|^p is L1. So apply Fubini to the function |f(x,y)|^p, which is L^1.
 
Thanks a lot for help!
 

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