Lebesgue Integral in QM: Tutorial & Reference

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Discussion Overview

The discussion centers around the Lebesgue integral in the context of quantum mechanics (QM), particularly its mathematical formalism related to Hilbert spaces. Participants explore the differences between Lebesgue and Riemann integrals, their applications in QM, and seek resources for better understanding.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant requests a tutorial or reference on the Lebesgue integral, indicating a limited mathematical background.
  • Another participant notes that Lebesgue integration is more general and based on measure theory, suggesting that the differences may not significantly impact understanding QM.
  • It is mentioned that in Hilbert space, functions differing on a set of null measure are considered equivalent, which may not apply to distributions like delta functions.
  • A participant summarizes key points about Lebesgue integration, including the division of the y-axis for integration and the consistency of wave function integration across equivalent systems, while questioning if there is more to understand in physical terms.
  • One participant asserts that whenever a function is Riemann integrable, the Lebesgue integral yields the same result, emphasizing the broader applicability of the Lebesgue integral.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement regarding the applicability and implications of Lebesgue versus Riemann integration in QM. There is no consensus on the necessity of Lebesgue integration for all applications in QM, and some participants suggest that Riemann integration may suffice in many cases.

Contextual Notes

Participants highlight limitations in their mathematical backgrounds and the potential complexity of the topics discussed, including the treatment of distributions and the implications of measure theory.

Who May Find This Useful

Individuals studying quantum mechanics with a focus on mathematical formalism, particularly those interested in the foundational aspects of integration in physics.

teddy
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hi
can anyone give me a short tutorial on lebesgue integral in QM.
I am doing my first course in QM and got stuck in the mathematical formalism of Hilbert space .Even a good reference on the web will do.
my math back ground is only upto Reimann(spelling?) integral.
thanks

bye.
 
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The main difference between Lebesgue and Riemann from a user perspective is that Lebesgue integration is more general, based on measure theory. For a physicist, I don't believe there is enough of a difference as far as understanding QM.

If you want to get some quick background try using google and search for "Lebesgue". You will get a lot of good hits.
 
As for Hilbert space L^2 stands, the whole point is that two functions that are different in a set of null measure are to be considered the same function.

This does not apply to distributions (ie delta functions and pure waves), which do not live in Hilbert space, although they are used as a "rigged" structure over it.

I believe that for most aplications the imaginery of Riemannian integration is enough, even if the rigour asks for Lebesgue.
 
thanks.

On going thru your replies and the web i got the following points :

1) in Lebesgue integ we divide the y-axis into small intervals instead of x-axis for integration.

2)the lebesgue integration of wave-fn for two physically equivalent systems is always same while riemann integration may differ due to 1)

3)For distribution functions we use Dirac measure instead of lebesgue.

am i missing something ?
is it more to it,speaking in physical terms ?
 
Actually, whenever a function is Riemann integrable, the Lesbegue integral gives the same result. The reason the Lesbegue integral is "better" is because the Lesbegue integral works for a much vaster set of functions than the Riemann integral.

Hurkyl
 

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