The product of 2 infinite sums

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Discussion Overview

The discussion revolves around the product of two infinite sums represented by exponential functions, specifically examining the expression e^{ix} e^{-ix} and how to derive the result of 1 from the infinite series representation without prior knowledge of the exponential identity.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about deriving the result of the product of two exponentials as infinite series, specifically stating the series as ΣnΣm (x^n/(n!) (-x)^m/(m!)).
  • Another participant suggests not ignoring the imaginary unit i and grouping the products by degree of x to clarify the series expansion.
  • A participant reiterates the original question about deriving the result from the infinite series, acknowledging the omission of i in their previous message.
  • Some participants propose using the Euler's formula e^{ix} = cos(x) + i sin(x) and its symmetry properties as an alternative approach, though this is noted to sidestep the original question regarding the product of the infinite series.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to derive the result from the infinite series, with multiple viewpoints and approaches being discussed without resolution.

Contextual Notes

There are limitations regarding the handling of the imaginary unit i in the series expansion, and the discussion does not resolve the mathematical steps necessary to derive the product of the two infinite sums.

dyn
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Hi.
I know that eixe-ix = 1 but if I write the product of the 2 exponentials as infinite series I get
ΣnΣm xn/(n!) (-x)m/(m!)
without knowing the result is 1 using exponentials how would I get the result of this product of 2 infinite sums ?
Thanks
 
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What do you get, if you do not ignore the ##i## as you did, and group the products by degree of ##x##?
 
dyn said:
Hi.
I know that eixe-ix = 1 but if I write the product of the 2 exponentials as infinite series I get
ΣnΣm xn/(n!) (-x)m/(m!)
without knowing the result is 1 using exponentials how would I get the result of this product of 2 infinite sums ?
Thanks
So the series for eu is Σn un/(n!)
and u is either ix or -ix (in your example). But you lost the i when converting to the series.
 
Yes sorry I forgot the i when writing out the infinite series
 
You may also use: ##e^{ix}=(cosx+isinx)## and symmetry properties of ##sinx, cosx##.
 
WWGD said:
You may also use: ##e^{ix}=(cosx+isinx)## and symmetry properties of ##sinx, cosx##.
Sure, you could do that for this particular problem, but this ignores the OP's question about the product of two infinite series.
 

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