Is Fubini's Theorem Always Valid?

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SUMMARY

Fubini's Theorem is not universally valid; it holds under specific conditions, particularly when the limits of integration can be interchanged due to uniform convergence. The discussion highlights that while the mixed partial derivatives of a function, represented as ##\frac{\partial^2 f}{\partial y \partial x}## and ##\frac{\partial^2 f}{\partial x \partial y}##, are equal under certain conditions, the double integrals ##\int \int f\;dx dy## and ##\int \int f\;dy dx## are not always interchangeable. Understanding these conditions is crucial for proper application of Fubini's Theorem in mathematical analysis.

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Jhenrique
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I remember a teacher speaking that the fubini's theorem is valid under certain conditions. Implying that not is valid under others. I didn't understood exactly what this teacher was speaking, but the doubt remains still today. We know that ##\frac{\partial^2 f}{\partial y \partial x}## is always equal to ##\frac{\partial^2 f}{\partial x \partial y}##, unless that proofs the contrary. The same way, ##\int \int f\;dx dy## is unconditionally equals to ##\int \int f\;dy dx##?
 
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Neither of those are true in general. A commonly given sufficient condition is that limits can be interchanged if the convergence is uniform. That being many times the conditions are met.
 

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