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29 Lamé FunctionsLamé Polynomials

§29.15 Fourier Series and Chebyshev Series

Contents
  1. §29.15(i) Fourier Coefficients
  2. §29.15(ii) Chebyshev Series

§29.15(i) Fourier Coefficients

Polynomial \mathit{uE}^{m}_{2n}\left(z,k^{2}\right)

When \nu=2n, m=0,1,\dots,n, the Fourier series (29.6.1) terminates:

A convenient way of constructing the coefficients, together with the eigenvalues, is as follows. Equations (29.6.4), with p=1,2,\dots,n, (29.6.3), and A_{2n+2}=0 can be cast as an algebraic eigenvalue problem in the following way. Let

29.15.2 \mathbf{M}=\begin{bmatrix}\beta_{0}&\alpha_{0}&0&\cdots&0\\
\gamma_{1}&\beta_{1}&\alpha_{1}&\ddots&\vdots\\
0&\ddots&\ddots&\ddots&0\\
\vdots&\ddots&\gamma_{n-1}&\beta_{n-1}&\alpha_{n-1}\\
0&\cdots&0&\gamma_{n}&\beta_{n}\end{bmatrix}

be the tridiagonal matrix with \alpha_{p}, \beta_{p}, \gamma_{p} as in (29.3.11), (29.3.12). Let the eigenvalues of \mathbf{M} be H_{p} with

29.15.3 H_{0}<H_{1}<\cdots<H_{n},

and also let

29.15.4 [A_{0},A_{2},\dots,A_{2n}]^{\mathrm{T}}

be the eigenvector corresponding to H_{m} and normalized so that

and

Then

and (29.15.1) applies, with \phi again defined as in (29.2.5).

Polynomial \mathit{sE}^{m}_{2n+1}\left(z,k^{2}\right)

When \nu=2n+1, m=0,1,\dots,n, the Fourier series (29.6.16) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.3.13), (29.3.14). Also, replace (29.15.4), (29.15.5), (29.15.6) by

29.15.9 [A_{1},A_{3},\dots,A_{2n+1}]^{\mathrm{T}},
29.15.10 \sum_{p=0}^{n}A_{2p+1}^{2}=1,
29.15.11 \sum_{p=0}^{n}A_{2p+1}>0.

Then

and (29.15.8) applies.

Polynomial \mathit{cE}^{m}_{2n+1}\left(z,k^{2}\right)

When \nu=2n+1, m=0,1,\dots,n, the Fourier series (29.6.31) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.3.15), (29.3.16). Also, replace (29.15.4), (29.15.5), (29.15.6) by

29.15.14 [B_{1},B_{3},\dots,B_{2n+1}]^{\mathrm{T}},
29.15.15 \sum_{p=0}^{n}B_{2p+1}^{2}=1,
29.15.16 \sum_{p=0}^{n}(2p+1)B_{2p+1}>0.

Then

and (29.15.13) applies.

Polynomial \mathit{dE}^{m}_{2n+1}\left(z,k^{2}\right)

When \nu=2n+1, m=0,1,\dots,n, the Fourier series (29.6.8) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.6.11). Also, replace (29.15.4), (29.15.5), (29.15.6) by

29.15.19 [C_{0},C_{2},\dots,C_{2n}]^{\mathrm{T}},
29.15.20 \left(1-\tfrac{1}{2}k^{2}\right)\left(\tfrac{1}{2}C_{0}^{2}+\sum_{p=1}^{n}C_{2%
p}^{2}\right)-\tfrac{1}{2}k^{2}\sum_{p=0}^{n-1}C_{2p}C_{2p+2}=1,
29.15.21 \tfrac{1}{2}C_{0}+\sum_{p=1}^{n}C_{2p}>0.

Then

and (29.15.18) applies.

Polynomial \mathit{scE}^{m}_{2n+2}\left(z,k^{2}\right)

When \nu=2n+2, m=0,1,\dots,n, the Fourier series (29.6.46) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.3.17). Also replace (29.15.4), (29.15.5), (29.15.6) by

29.15.24 [B_{2},B_{4},\dots,B_{2n+2}]^{\mathrm{T}},
29.15.25 \sum_{p=0}^{n}B_{2p+2}^{2}=1,
29.15.26 \sum_{p=0}^{n}(2p+2)B_{2p+2}>0.

Then

and (29.15.23) applies.

Polynomial \mathit{sdE}^{m}_{2n+2}\left(z,k^{2}\right)

When \nu=2n+2, m=0,1,\dots,n, the Fourier series (29.6.23) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.6.26). Also replace (29.15.4), (29.15.5), (29.15.6) by

29.15.29 [C_{1},C_{3},\dots,C_{2n+1}]^{\mathrm{T}},
29.15.30 \left(1-\tfrac{1}{2}k^{2}\right)\sum_{p=0}^{n}C_{2p+1}^{2}-{\tfrac{1}{2}k^{2}%
\left(\tfrac{1}{2}C_{1}^{2}+\sum_{p=0}^{n-1}C_{2p+1}C_{2p+3}\right)=1},
29.15.31 \sum_{p=0}^{n}C_{2p+1}>0.

Then

and (29.15.28) applies.

Polynomial \mathit{cdE}^{m}_{2n+2}\left(z,k^{2}\right)

When \nu=2n+2, m=0,1,\dots,n, the Fourier series (29.6.38) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.6.41). Also replace (29.15.4), (29.15.5), (29.15.6) by

29.15.34 [D_{1},D_{3},\dots,D_{2n+1}]^{\mathrm{T}},
29.15.35 \left(1-\tfrac{1}{2}k^{2}\right)\sum_{p=0}^{n}D_{2p+1}^{2}+{\tfrac{1}{2}k^{2}%
\left(\tfrac{1}{2}D_{1}^{2}-\sum_{p=0}^{n-1}D_{2p+1}D_{2p+3}\right)=1},
29.15.36 \sum_{p=0}^{n}(2p+1)D_{2p+1}>0.

Then

and (29.15.33) applies.

Polynomial \mathit{scdE}^{m}_{2n+3}\left(z,k^{2}\right)

When \nu=2n+3, m=0,1,\dots,n, the Fourier series (29.6.53) terminates:

In (29.15.2) replace \alpha_{p}, \beta_{p}, and \gamma_{p} as in (29.6.56). Also replace (29.15.4), (29.15.5), (29.15.6) by

29.15.39 [D_{2},D_{4},\dots,D_{2n+2}]^{\mathrm{T}},
29.15.40 \left(1-\tfrac{1}{2}k^{2}\right)\sum_{p=0}^{n}D_{2p+2}^{2}-\tfrac{1}{2}k^{2}%
\sum_{p=1}^{n}D_{2p}D_{2p+2}=1,
29.15.41 \sum_{p=0}^{n}(2p+2)D_{2p+2}>0.

Then

and (29.15.38) applies.

§29.15(ii) Chebyshev Series

The Chebyshev polynomial T of the first kind (§18.3) satisfies \cos\left(p\phi\right)=T_{p}\left(\cos\phi\right). Since (29.2.5) implies that \cos\phi=\operatorname{sn}\left(z,k\right), (29.15.1) can be rewritten in the form

This determines the polynomial P of degree n for which \mathit{uE}^{m}_{2n}\left(z,k^{2}\right)=P({\operatorname{sn}}^{2}\left(z,k%
\right)); compare Table 29.12.1. The set of coefficients of this polynomial (without normalization) can also be found directly as an eigenvector of an (n+1)\times(n+1) tridiagonal matrix; see Arscott and Khabaza (1962).

Using also \sin\left((p+1)\phi\right)=(\sin\phi)U_{p}\left(\cos\phi\right), with U denoting the Chebyshev polynomial of the second kind (§18.3), we obtain

For explicit formulas for Lamé polynomials of low degree, see Arscott (1964b, p. 205).