I Equality of integrals VS equality of integrands

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The discussion centers on whether the equality of integrals, specifically $$\int_{0}^{\infty}f(t)dt=\int_{0}^{\infty}g(t)dt$$, implies that the functions themselves are equal, $$f(t)=g(t)$$. Participants argue that this is false, providing examples of different functions that yield the same integral value. They highlight that integrable functions can differ at specific points while maintaining equal integrals, and that the linear functional representing the integral is not injective in infinite-dimensional spaces. The conversation also touches on conditions under which integrals can imply equality of functions, particularly for non-negative continuous functions. Overall, the consensus is that equal integrals do not guarantee equal integrands.
  • #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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