Solving Summation of sin with n^2 - Svensl

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Discussion Overview

The discussion revolves around the summation of the sine function with a quadratic term in the exponent, specifically the expression \(\sum_{n=0}^{K-1}\frac{sin(2\pi n^2\Delta)}{n}\). Participants explore potential methods for solving this summation, considering both theoretical and mathematical approaches.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant, svensl, seeks hints for solving the summation and attempts to rewrite it using complex exponentials, indicating difficulty with the \(n^2\) term.
  • Another participant questions the nature of \(\Delta\), suggesting that if it were an integer, the sine function would evaluate to zero for integer multiples of \(2\pi\).
  • svensl clarifies that \(\Delta\) is a number between 0 and 1 and notes that \(K\) will eventually approach infinity, which may impact the solution.
  • A further suggestion is made regarding the use of a function with poles in the complex plane, proposing that contour integration and Jordan's lemma might transform the sum into an integral along the real line.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific method for solving the summation, and multiple approaches are proposed without resolution.

Contextual Notes

There are assumptions regarding the nature of \(\Delta\) and the implications of taking \(K\) to infinity that remain unresolved. The discussion also lacks a clear mathematical framework for the proposed methods.

svensl
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Hello,
Can anyone give some hints on how to solve this:

[tex]\sum_{n=0}^{K-1}\frac{sin(2\pi n^2\Delta)}{n}[/tex]

It's just the n^2 that complicates things. I tried re-writing it as

[tex]Im\sum_{n=0}^{K-1}\frac{e^{j n^2 x}}{n}[/tex],

where [tex]x=2\pi \Delta[/tex]
but I cannot solve this either.

Thanks,
svensl
 
Last edited:
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What is delta? If it is an integer than sin(2*pi*k) for any integer k is equal to 0.
 
Thanks for the reply.

Delta is a number between (0, 1(.
BTW, K will later be taken to infinity if that makes a difference.
 
Perhaps some well choosen function which has poles at certain places in the complex plane to give that summation as residues might be useful? Then you can use a contour integral and Jordans lemma to turn that sum into an integral along the Reals somehow?

That's without putting pen to paper so I might be way off.
 

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