What Type of Expansion is Used Here?

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Discussion Overview

The discussion revolves around the type of mathematical expansion used in the context of Rayleigh instability, specifically focusing on a series expansion related to the expression involving a square root and a trigonometric function. The scope includes mathematical reasoning and exploration of series expansions.

Discussion Character

  • Mathematical reasoning, Technical explanation

Main Points Raised

  • One participant identifies the expansion as the Binomial series expansion of ##(1+x)^{1/2}##.
  • Another participant suggests that if ##f(x) = (\frac{2\pi\delta}{\lambda})^2 \cos^2(\frac{2\pi x}{\lambda})##, then it may be possible to derive a Taylor Expansion by taking the derivative with respect to ##f(x)##.
  • A different participant clarifies the general form of the Binomial expansion, indicating that it applies where ##x## is the trigonometric function and ##n = 1/2##.
  • One participant expresses initial confusion but later indicates understanding after the clarification.

Areas of Agreement / Disagreement

Participants generally agree on the identification of the expansion as a Binomial series, but there is some uncertainty regarding the application of derivatives and Taylor expansions in this context. The discussion remains somewhat unresolved regarding the implications of these expansions.

Contextual Notes

There are limitations in the assumptions made about the function ##f(x)## and its derivatives, as well as the specific conditions under which the expansions are valid.

Chuck88
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When I am reading the paper about Rayleigh instability, I found this type of expanding method.

[tex] \sqrt{1+(\frac{2\pi\delta}{\lambda})^2 \cos^2(\frac{2\pi x}{\lambda})} = 1 + \frac{1}{2}(\frac{2\pi\delta}{\lambda})^2\cos^2 (\frac{2\pi x}{\lambda}) + \cdots[/tex]

Can someone tell me what type of expansion is this?
 
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It's the Binomial series expansion of ##(1+x)^{1/2}##
 
AlephZero said:
It's the Binomial series expansion of ##(1+x)^{1/2}##

If we suppose that ##f(x) = (\frac{2\pi\delta}{\lambda})^2 \cos^2(\frac{2\pi x}{\lambda})##, is it right that we could take derivative with respect to ##f(x)## to get the Taylor Expansion? The first order derivative I mean is presented below:

[tex] \frac{d(1 + f(x))^{\frac{1}{2}}}{df(x)} = \frac{1}{2} \frac{1}{\sqrt{1 + f(x)}}[/tex]
 
I'm not sure where that is leading to. I meant
##(1+x)^n = 1 + n x + n(n-1)x^2 / 2! + n(n-1)(n-2)x^3/3! + \dots##
where x is the trig function and n = 1/2.
 
AlephZero said:
I'm not sure where that is leading to. I meant
##(1+x)^n = 1 + n x + n(n-1)x^2 / 2! + n(n-1)(n-2)x^3/3! + \dots##
where x is the trig function and n = 1/2.

OK. Now I comprehend. Thanks a lot.
 

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