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Legendre Polynomial (anti)symmetry proof

  • Thread starter Tetrinity
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  • #1
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Homework Statement



Let [itex]P_{n}(x)[/itex] denote the Legendre polynomial of degree n, n = 0, 1, 2, ... . Using the formula for the generating function for the sequence of Legendre polynomials, show that:

[itex]P_{n}(-x) = (-1)^{n}P_{n}(x)[/itex]

for any x [itex]\in[/itex] [-1, 1], n = 0, 1, 2, ... .

Homework Equations



Generating function for the sequence of Legendre polynomials:

[itex]\sum P_{n}(x)r^{n} = (1 - 2rx + r^{2})^{-\frac{1}{2}}[/itex]

The Attempt at a Solution



I guess I don't really know where to begin with this. I tried differentiating both sides of the generating function with respect to r to obtain:

[itex]\sum nP_{n}(x)r^{n-1} = (x - r)\sum P_{n}(x)r^{n}[/itex]

but I don't see how this might move closer to the desired result. I imagine differentiating with respect to x would lead to similar difficulties.


I then tried just directly substituting -x into the generating function, giving

[itex](1 + 2rx + r^{2})^{-\frac{1}{2}}[/itex]

as a generating function for [itex]P_{n}(-x)[/itex], but again I don't really see where this might lead me.

Perhaps there's a general method I'm missing; I'm still very new to the concept of Legendre polynomials and generating functions in general. My attempts at scouring the Internet for a proof of this specific identity have turned up a sole reference to a textbook I have no access to. I'm not really looking for a complete proof to be given to me on a plate, but a nudge in the right direction would certainly be appreciated.
 

Answers and Replies

  • #2
Dick
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Homework Helper
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Try substituting both -x for x and -r for r into the generating function expression. What does that tell you?
 
  • #3
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Ahh, figures it'd be something as simple as that.

[itex]\sum P_{n}(-x)(-r)^{n} = (1 + 2rx + r^{2})^{\frac{1}{2}} = \sum P_{n}(x)r^{n}[/itex]

[itex]\Rightarrow \sum P_{n}(x)r^{n} = \sum P_{n}(-x)(-1)^{n}r^{n}[/itex]

and then by applying the identity theorem for power series we get [itex]P_{n}(x) = (-1)^{n}P_{n}(x)[/itex] as required.

Thanks for your help. :)
 

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