Digital Library of Mathematical Functions
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29 Lamé FunctionsLamé Polynomials

§29.15 Fourier Series and Chebyshev Series

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§29.15(i) Fourier Coefficients

Polynomial \mathop{\mathit{uE}^{{m}}_{{2n}}\/}\nolimits\!\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

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

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

and also let

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).

§29.15(ii) Chebyshev Series

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

This determines the polynomial P of degree n for which \mathop{\mathit{uE}^{{m}}_{{2n}}\/}\nolimits\!\left(z,k^{2}\right)=P({\mathop{%
\mathrm{sn}\/}\nolimits^{{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 \mathop{\sin\/}\nolimits\!\left((p+1)\phi\right)=(\mathop{\sin\/}\nolimits\phi%
)\mathop{U_{{p}}\/}\nolimits\!\left(\mathop{\cos\/}\nolimits\phi\right), with \mathop{U\/}\nolimits 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).