Opinion about an introduction of the Lebesgue integral in "Introduction to Hilbert spaces with applications"

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Discussion Overview

The discussion centers around the introduction of the Lebesgue integral in the book "Introduction to Hilbert spaces with applications" by Debnath and Mikusinski. Participants express their opinions on the validity of the method used in the book, particularly in the context of the author's avoidance of measure theory and the use of step functions. The scope includes theoretical aspects of Lebesgue integration and its pedagogical implications for students with limited mathematical backgrounds.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant expresses uncertainty about the correctness of the book's method for introducing the Lebesgue integral without measure theory, seeking opinions from others who have read the book.
  • Another participant questions the meaning of "correct" in this context, implying that there may be errors in the book.
  • A further response clarifies that the approach using step functions is valid and standard, suggesting that the method is acceptable despite not following the traditional measure theory route.
  • Another participant elaborates on two approaches to Lebesgue integrals, contrasting the measure theory approach with a limit-based approach, asserting that both are valid and that the limit approach can enhance understanding.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the validity of the book's method. While some argue that the approach is standard and correct, others express skepticism and seek clarification on the implications of avoiding measure theory.

Contextual Notes

Some participants highlight the limitations of the book's approach, particularly for readers without a background in measure theory, and the potential for misunderstanding the development of the Lebesgue integral.

Cauer
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I am an undergraduate student in physics. I wanted to deepen in the theory of Hilbert spaces and I took the book "Introduction to Hilbert spaces with applications" of Debnath and Mikusinski. I am writing this thread to know the opinion of the readers about the method of introducing the Lebesgue integral that one can find in the above-mentioned book. I don't have knowledgement of measure theory. The program in the bachelor's degree in physics is so short in the mathematical aspect! For someone who has worked the book, is "correct" the method for arriving to the Lebesgue integral?

Regards,
 
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Cauer said:
I am an undergraduate student in physics. I wanted to deepen in the theory of Hilbert spaces and I took the book "Introduction to Hilbert spaces with applications" of Debnath and Mikusinski. I am writing this thread to know the opinion of the readers about the method of introducing the Lebesgue integral that one can find in the above-mentioned book. I don't have knowledgement of measure theory. The program in the bachelor's degree in physics is so short in the mathematical aspect! For someone who has worked the book, is "correct" the method for arriving to the Lebesgue integral?

Regards,
While you are waiting for someone who has exactly this one book, you could read
https://www.physicsforums.com/insights/omissions-mathematics-education-gauge-integration/
 
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Cauer said:
I am an undergraduate student in physics. I wanted to deepen in the theory of Hilbert spaces and I took the book "Introduction to Hilbert spaces with applications" of Debnath and Mikusinski. I am writing this thread to know the opinion of the readers about the method of introducing the Lebesgue integral that one can find in the above-mentioned book. I don't have knowledgement of measure theory. The program in the bachelor's degree in physics is so short in the mathematical aspect! For someone who has worked the book, is "correct" the method for arriving to the Lebesgue integral?

Regards,
What do you mean by "correct"? Do you think that there are errors in the book?
 
I mean that I don't know if the development made in the book to arrive to the Lebesgue integral by using step functions and avoiding the theory of measure is correct. Is the approach made by the book authors as valid as the method that starts with the concept of measure and then defines the integral?
 
Cauer said:
I mean that I don't know if the development made in the book to arrive to the Lebesgue integral by using step functions and avoiding the theory of measure is correct. Is the approach made by the book authors as valid as the method that starts with the concept of measure and then defines the integral?
So you think that the book was published even though it may have been incorrect and using a methid that is not valid!

Anyway, no, you dont need to worry. The approach is correct and standard.
 
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there are two ways to approach lebesgue integrals. one is to treat measure theory and define lebesgue integrals as limits of sums involving measures of complicated sets where a function has values in a given interval. then one proves that in this theory, limits of functions behave well, in the sense that reasonable limits of integrable functions are again integrable, and the integral of the limit is the limit of the integrals.

After the fact, it is clear then that, since limits of integrable functions are lebesgue integrable, we could have just defined lebesgue integrable functions to be such limits. Indeed this approach helped me appreciate lebesgue integration after years of struggling with the complexity of the other definition.

the limit approach is certainly valid and rigorous and is adopted in several very highly regarded books, such as Functional Analysis, by Riesz-Nagy, and Foundations of modern analysis, by Dieudonne'.
 

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