When
,
, 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
, (29.6.3), and
can be cast as an
algebraic eigenvalue problem in the following way. Let
be the tridiagonal matrix with
,
,
as in
(29.3.11), (29.3.12).
Let the eigenvalues of
be
with
and also let
be the eigenvector corresponding to
and normalized so that
and
Then
The Chebyshev polynomial
of the first kind (§18.3)
satisfies
. Since
(29.2.5) implies that
,
(29.15.1) can be rewritten in the form
This determines the polynomial
of degree
for which
; compare Table
29.12.1. The set of coefficients of this polynomial (without
normalization) can also be found directly as an eigenvector of an
tridiagonal matrix; see Arscott and Khabaza (1962).
Using also
, with
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).