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
When
,
, the Fourier series (29.6.16)
terminates:
In (29.15.2) replace
,
, and
as in
(29.3.13), (29.3.14). Also, replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.8) applies.
When
,
, the Fourier series (29.6.31)
terminates:
In (29.15.2) replace
,
, and
as in
(29.3.15), (29.3.16). Also, replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.13) applies.
When
,
, the Fourier series (29.6.8)
terminates:
In (29.15.2) replace
,
, and
as in
(29.6.11). Also, replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.18) applies.
When
,
, the Fourier series (29.6.46)
terminates:
In (29.15.2) replace
,
, and
as in
(29.3.17). Also replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.23) applies.
When
,
, the Fourier series (29.6.23)
terminates:
In (29.15.2) replace
,
, and
as in
(29.6.26). Also replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.28) applies.
When
,
, the Fourier series (29.6.38)
terminates:
In (29.15.2) replace
,
, and
as in
(29.6.41). Also replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.33) applies.
When
,
, the Fourier series (29.6.53)
terminates:
In (29.15.2) replace
,
, and
as in
(29.6.56). Also replace
(29.15.4), (29.15.5), (29.15.6) by
Then
and (29.15.38) applies.
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).