Simplifying a double summation

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    Simplifying Summation
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Discussion Overview

The discussion revolves around the simplification of a double summation function involving error functions and constants. Participants explore whether it is feasible to eliminate the summations entirely and what methods might be employed to achieve this.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the expectation of reducing the complex function to a no-sum form, expressing uncertainty about the feasibility of such simplification.
  • Another participant suggests removing constants from the sums to simplify the expression, indicating that these constants complicate the function unnecessarily.
  • A further suggestion is made to express the error functions as integrals to investigate potential relationships between adjacent terms that could facilitate summation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the possibility of simplifying the summation. Multiple views on the complexity and potential methods for simplification are presented.

Contextual Notes

The discussion highlights the complexity of the function and the potential for various methods of simplification, but does not resolve the mathematical challenges involved.

Ad VanderVen
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TL;DR
Simplifying a double summation.
Is it possible to simplify the function below so that the sums disappear.
$$\displaystyle g \left(x \right) \, = \, \sum _{j=-\infty}^{\infty} \left(-A +B \right) \sum _{k=-\infty}^{\infty} \frac{1}{2}~\frac{\sqrt{2}~e^{-\frac{1}{2}~\frac{\left(x -k \right)^{2}}{\sigma ^{2}}}~\left(U -V \right)}{\sigma ~\sqrt{\pi }}$$
with
$$\displaystyle A\, = \,1/2\,{\rm erf} \left(1/2\,{\frac { \sqrt{2} \left( -j-1/2+{\it omicron} \right) }{\rho}}\right),$$
$$\displaystyle B\, = \,1/2\,{\rm erf} \left(1/2\,{\frac { \sqrt{2} \left( -j+1/2+{\it omicron} \right) }{\rho}}\right),$$
$$\displaystyle U\, = \,1/2\,{\rm erf} \left(1/4\,{\frac { \sqrt{2} \left( -2\,bj+2\,k+1 \right) }{\tau}}\right)$$
and
$$\displaystyle V\, = \,1/2\,{\rm erf} \left(1/4\,{\frac { \sqrt{2} \left( -2\,bj+2\,k-1 \right) }{\tau}}\right)$$
 
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It seems very complex to me. Why do you estimate or expect that it would be reduced to a no sum form ?
 
anuttarasammyak said:
It seems very complex to me. Why do you estimate or expect that it would be reduced to a no sum form ?
I do not know.
 
Get rid of all the constants that can be taken out of the sums, they just blow up the expression for absolutely no reason.

You can express all the error functions as integrals and then see if adjacent terms have some nice relation for the boundaries that allows summation. The error function arguments look like there might be something you can combine.
 

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