When we can change a sum to an integral?

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Discussion Overview

The discussion revolves around the conditions under which a sum can be approximated or transformed into an integral, particularly in the context of physics. Participants explore theoretical and conceptual aspects of this transformation, including specific scenarios and examples.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that a sum can be changed to an integral when the indexing value becomes continuous, such as transitioning from discrete values to a continuous variable like x.
  • Another participant notes that an integral represents the limit of a sum of an arbitrarily large number of arbitrarily small elements.
  • A concern is raised about the validity of changing a sum to an integral when one or more terms in the sum are significantly larger than the others, implying that such terms may not be considered small elements.
  • One participant illustrates the concept by discussing how sums can approximate the area under a curve, emphasizing that as the width of the boxes used in the approximation decreases, the sum approaches an integral.
  • A question is posed regarding the treatment of particles in the ground state during Bose-Einstein condensation, suggesting that the large number of particles in the ground state may necessitate separating them from the integral.

Areas of Agreement / Disagreement

Participants express differing views on the conditions for transforming sums into integrals, with no consensus reached on specific criteria or examples. The discussion remains unresolved regarding the implications of large terms in sums and their relation to integrals.

Contextual Notes

Limitations include the lack of clarity on the specific mathematical conditions required for the transformation and the dependence on the definitions of terms involved in the discussion.

fxdung
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In physics we often change a sum to an integral.But I am not clear when can we change a sum to an integral?When a term of sum is comparable to the sum,can we change the sum to integral?
 
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When the indexing value i of the sum becomes continuous like x .
 
An integral is essentially the limit of a sum an arbitrarily large number of arbitrarily small elements.

AM
 
Then if one(or more) term of sum is very great(can not consider as a small element) so we can not consider as an integral?
 
Think about how you initially use sums to approximate the area under a curve.

you might compute the area as the sum of box areas where the width of the box is one and the length of the box is f(x) and use the sum of boxes with
x=1, 2, 3, 4, 5...

then to get a better approximation you use a width of one half for the box and the sequence with x=0.5, 1, 1.5, 2, 2.5 ...

so as delta x (aka box width and x difference) gets smaller and the x sequence becomes more continuous then an integral comes into play.
 
I do not understand why in Bose-Einstein condensation, below critical temprature we must separate the number of particles of ground state(E=0) from the integral?Is that because the number of particles in ground state become very large so that we must separate it from the integral(the total number of particles)?
 
You should ask that question in a separate thread with a related title As now you’re talking more physics and not math.

closing this thread now.
 

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