Looking for references on this form of a Taylor series

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Discussion Overview

The discussion revolves around a specific form of the Taylor series expressed in terms of the gradient operator and its application. Participants explore its validity, seek references, and discuss related mathematical concepts, including the conditions under which the series holds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant seeks confirmation of the validity of the Taylor series form: $$\vec f(\vec x+\vec a) = \sum_{n=0}^{\infty}\frac{1}{n!}(\vec a \cdot \nabla)^n\vec f(\vec x)$$ and requests references.
  • Another participant suggests that expressing the function as a 3D Fourier transform supports the validity of the series but questions whether this approach is unnecessarily restrictive.
  • A participant provides a reference to a document that contains the formula and emphasizes the need for the function to be analytic for the series to be applicable, noting that it aligns with the conventional Taylor's theorem upon expanding the operator.
  • The same participant expresses appreciation for the resource and mentions the usefulness of "multi-index" notation in this context.

Areas of Agreement / Disagreement

Participants generally agree on the need for the function to be analytic for the series to be valid. However, there is no consensus on the implications of using a 3D Fourier transform or the potential restrictions it may impose.

Contextual Notes

The discussion highlights the assumption that the function must be analytic for the Taylor series to hold, but does not resolve the implications of this requirement or the nature of the Fourier transform's role.

Hiero
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I was trying to find this form of the Taylor series online:
$$\vec f(\vec x+\vec a) = \sum_{n=0}^{\infty}\frac{1}{n!}(\vec a \cdot \nabla)^n\vec f(\vec x)$$
But I can’t find it anywhere. Can someone confirm it’s validity and/or provide any links which mention it? It seems quite powerful to be so obscure; maybe I’m just bad at googling.

Thanks.
 
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It is clear that expressing f(x) as a 3D Fourier transform shows this to be true but I don't know whether that is unnecessarily restrictive.
 
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I found your formula here in the first line of page 3: https://sites.math.washington.edu/~folland/Math425/taylor2.pdf. You should of course assume that your function is actually analytic for this to make sense (not using a finite sum and error term).

It's equivalent to the usual formulation of Taylor's theorem once you expand out what ##(a\cdot\nabla)^n## means and do the combinatorics.
 
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Infrared said:
I found your formula here in the first line of page 3: https://sites.math.washington.edu/~folland/Math425/taylor2.pdf. You should of course assume that your function is actually analytic for this to make sense (not using a finite sum and error term).

It's equivalent to the usual formulation of Taylor's theorem once you expand out what ##(a\cdot\nabla)^n## means and do the combinatorics.
Thank you! Great resource. I had never seen “multi-index” notation before; it’s very useful for this purpose!

I’m impressed by how quickly you found that. I guess I am just bad with google o:)

Take care
 

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