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28 Mathieu Functions and Hill’s EquationMathieu Functions of Integer Order

§28.4 Fourier Series

Contents
  1. §28.4(i) Definitions
  2. §28.4(ii) Recurrence Relations
  3. §28.4(iii) Normalization
  4. §28.4(iv) Case q=0
  5. §28.4(v) Change of Sign of q
  6. §28.4(vi) Behavior for Small q
  7. §28.4(vii) Asymptotic Forms for Large m

§28.4(i) Definitions

The Fourier series of the periodic Mathieu functions converge absolutely and uniformly on all compact sets in the z-plane. For n=0,1,2,3,\dots,

28.4.1 \operatorname{ce}_{2n}\left(z,q\right)=\sum_{m=0}^{\infty}A^{2n}_{2m}(q)\cos 2mz,

§28.4(ii) Recurrence Relations

28.4.5
aA_{0}-qA_{2}=0,
(a-4)A_{2}-q(2A_{0}+A_{4})=0,
(a-4m^{2})A_{2m}-q(A_{2m-2}+A_{2m+2})=0, m=2,3,4,\dots, a=a_{2n}\left(q\right), A_{2m}=A_{2m}^{2n}(q).
28.4.6
(a-1-q)A_{1}-qA_{3}=0,
\left(a-(2m+1)^{2}\right)A_{2m+1}-q(A_{2m-1}+A_{2m+3})=0,m=1,2,3,\dots, a=a_{2n+1}\left(q\right), A_{2m+1}=A_{2m+1}^{2n+1}(q).
28.4.7
(a-1+q)B_{1}-qB_{3}=0,
\left(a-(2m+1)^{2}\right)B_{2m+1}-q(B_{2m-1}+B_{2m+3})=0,m=1,2,3,\dots, a=b_{2n+1}\left(q\right), B_{2m+1}=B_{2m+1}^{2n+1}(q).
28.4.8
(a-4)B_{2}-qB_{4}=0,
(a-4m^{2})B_{2m}-q(B_{2m-2}+B_{2m+2})=0, m=2,3,4,\dots, a=b_{2n+2}\left(q\right), B_{2m+2}=B_{2m+2}^{2n+2}(q).

§28.4(iii) Normalization

28.4.9 2\left(A^{2n}_{0}(q)\right)^{2}+\sum_{m=1}^{\infty}\left(A^{2n}_{2m}(q)\right)%
^{2}=1,
28.4.10 \sum_{m=0}^{\infty}\left(A^{2n+1}_{2m+1}(q)\right)^{2}=1,
28.4.11 \sum_{m=0}^{\infty}\left(B^{2n+1}_{2m+1}(q)\right)^{2}=1,
28.4.12 \sum_{m=0}^{\infty}\left(B^{2n+2}_{2m+2}(q)\right)^{2}=1.

Ambiguities in sign are resolved by (28.4.13)–(28.4.16) when q=0, and by continuity for the other values of q.

§28.4(iv) Case q=0

28.4.13
A^{0}_{0}(0)=1/\sqrt{2},\quad A^{2n}_{2n}(0)=1,n>0,
A^{2n}_{2m}(0)=0,n\neq m,
28.4.14
A^{2n+1}_{2n+1}(0)=1,
A^{2n+1}_{2m+1}(0)=0,n\neq m,
28.4.15
B^{2n+1}_{2n+1}(0)=1,
B^{2n+1}_{2m+1}(0)=0,n\neq m,
28.4.16
B^{2n+2}_{2n+2}(0)=1,
B^{2n+2}_{2m+2}(0)=0,n\neq m.

§28.4(v) Change of Sign of q

28.4.17 A^{2n}_{2m}(-q)=(-1)^{n-m}A^{2n}_{2m}(q),
28.4.18 B^{2n+2}_{2m+2}(-q)=(-1)^{n-m}B^{2n+2}_{2m+2}(q),
28.4.19 A^{2n+1}_{2m+1}(-q)=(-1)^{n-m}B^{2n+1}_{2m+1}(q),
28.4.20 B^{2n+1}_{2m+1}(-q)=(-1)^{n-m}A^{2n+1}_{2m+1}(q).

§28.4(vi) Behavior for Small q

For fixed s=1,2,3,\dots and fixed m=1,2,3,\dots,

28.4.21 A^{0}_{2s}(q)=\left(\dfrac{(-1)^{s}2}{(s!)^{2}}\left(\frac{q}{4}\right)^{s}+O%
\left(q^{s+2}\right)\right)A^{0}_{0}(q),

For further terms and expansions see Meixner and Schäfke (1954, p. 122) and McLachlan (1947, §3.33).

§28.4(vii) Asymptotic Forms for Large m

As m\to\infty, with fixed q (\neq 0) and fixed n,

For the basic solutions w_{\mbox{\tiny I}} and w_{\mbox{\tiny II}} see §28.2(ii).