Equality of integrals VS equality of integrands

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SUMMARY

The discussion centers on the relationship between the equality of integrals and the equality of integrands, specifically questioning whether $$\int_{0}^{\infty}f(t)dt=\int_{0}^{\infty}g(t)dt$$ implies $$f(t)=g(t)$$. Participants provided counterexamples, demonstrating that different functions can yield the same integral value. Key points include that if $$f \geq 0$$ and $$\int f = 0$$, then $$f = 0$$ almost everywhere, and if $$f$$ is continuous with $$\int f = 0$$, then $$f = 0$$ everywhere. The discussion also touches on functional derivatives in the context of calculus of variations.

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  • Understanding of Riemann integrals and their properties
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  • #31
AFAIK there are no problems up to here: $$\frac{d}{dx} \lim_{x \to \infty} \int_{0}^{x} \left[f(t) - g(t) \right] = 0$$ but yes I don't think you can generally interchange those limits to get ##\lim_{x \to \infty} \frac{d}{dx} \int_{0}^{x} \left[f(t) - g(t) \right] = 0##
 
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  • #32
Math_QED said:
so I'm not sure where your claim comes from.
People whom I know (in real life) learned by themselves, teachers only graded them.
 
  • #33
Adesh said:
People whom I know (in real life) learned by themselves, teachers only graded them.

My opinion is that most of mathematics is self-study. The professor is just there to motivate the concepts and give a first exposure, or to ask questions to.
 
  • #34
Math_QED said:
My opinion is that most of mathematics is self-study. The professor is just there to motivate the concepts and give a first exposure, or to ask questions to.
Then what does it mean when so many people say “Rudin is not good for self-study” ?
 
  • #35
Adesh said:
Then what does it mean when so many people say “Rudin is not good for self-study” ?

It means that it is a bad book to learn the material from, with which I agree. There are much better self-study books out there.
 
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  • #36
@Adesh I generally had very good professors in undergrad and learned a lot from their lectures. I can sometimes be a slow student though, so I generally had to re-work the parts I didn't follow afterwards, but this is all on me and certainly doesn't reflect negatively on my instructors.

I think Rudin is harder to self-study because it's terse and doesn't "hand-hold" the reader. This is much less of an obstacle when you have all the aspects of a class to help you along (other students, professors, teaching assistants, homeworks, etc.)

In the future, you might want to PM a user if you have a specific question for them, instead of posting on an unrelated thread.
 
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