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

§28.6 Expansions for Small q

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§28.6(i) Eigenvalues

Leading terms of the power series for \mathop{a_{{m}}\/}\nolimits\!\left(q\right) and \mathop{b_{{m}}\/}\nolimits\!\left(q\right) for m\leq 6 are:

28.6.1\mathop{a_{{0}}\/}\nolimits\!\left(q\right)=-\tfrac{1}{2}q^{2}+\tfrac{7}{128}q%
^{4}-\tfrac{29}{2304}q^{6}+\tfrac{68687}{188\;74368}q^{8}+\cdots,

Leading terms of the of the power series for m=7,8,9,\dots are:

The coefficients of the power series of \mathop{a_{{2n}}\/}\nolimits\!\left(q\right), \mathop{b_{{2n}}\/}\nolimits\!\left(q\right) and also \mathop{a_{{2n+1}}\/}\nolimits\!\left(q\right), \mathop{b_{{2n+1}}\/}\nolimits\!\left(q\right) are the same until the terms in q^{{2n-2}} and q^{{2n}}, respectively. Then

Higher coefficients in the foregoing series can be found by equating coefficients in the following continued-fraction equations:

Numerical values of the radii of convergence \rho_{n}^{{(j)}} of the power series (28.6.1)–(28.6.14) for n=0,1,\dots,9 are given in Table 28.6.1. Here j=1 for \mathop{a_{{2n}}\/}\nolimits\!\left(q\right), j=2 for \mathop{b_{{2n+2}}\/}\nolimits\!\left(q\right), and j=3 for \mathop{a_{{2n+1}}\/}\nolimits\!\left(q\right) and \mathop{b_{{2n+1}}\/}\nolimits\!\left(q\right). (Table 28.6.1 is reproduced from Meixner et al. (1980, §2.4).)

Table 28.6.1: Radii of convergence for power-series expansions of eigenvalues of Mathieu’s equation.
n \rho^{{(1)}}_{n} \rho^{{(2)}}_{n} \rho^{{(3)}}_{n}
0 or 1 1.46876 86138 6.92895 47588 3.76995 74940
2 7.26814 68935 16.80308 98254 11.27098 52655
3 16.47116 58923 30.09677 28376 22.85524 71216
4 30.42738 20960 48.13638 18593 38.52292 50099
5 47.80596 57026 69.59879 32769 58.27413 84472
6 69.92930 51764 95.80595 67052 82.10894 36067
7 95.47527 27072 125.43541 1314 110.02736 9210
8 125.76627 89677 159.81025 4642 142.02943 1279
9 159.47921 26694 197.60667 8692 178.11513 940

It is conjectured that for large n, the radii increase in proportion to the square of the eigenvalue number n; see Meixner et al. (1980, §2.4). It is known that

where k is the unique root of the equation 2\!\mathop{E\/}\nolimits\!\left(k\right)=\mathop{K\/}\nolimits\!\left(k\right) in the interval (0,1), and k^{{\prime}}=\sqrt{1-k^{2}}. For \mathop{E\/}\nolimits\!\left(k\right) and \mathop{K\/}\nolimits\!\left(k\right) see §19.2(ii).

§28.6(ii) Functions \mathop{\mathrm{ce}_{{n}}\/}\nolimits and \mathop{\mathrm{se}_{{n}}\/}\nolimits

Leading terms of the power series for the normalized functions are:

For m=3,4,5,\dots,

28.6.26\mathop{\mathrm{ce}_{{m}}\/}\nolimits\!\left(z,q\right)=\mathop{\cos\/}%
\nolimits mz-\frac{q}{4}\left(\frac{1}{m+1}\mathop{\cos\/}\nolimits(m+2)z-%
\frac{1}{m-1}\mathop{\cos\/}\nolimits(m-2)z\right)+\frac{q^{2}}{32}\left(\frac%
{1}{(m+1)(m+2)}\mathop{\cos\/}\nolimits(m+4)z+\frac{1}{(m-1)(m-2)}\mathop{\cos%
\/}\nolimits(m-4)z-\frac{2(m^{2}+1)}{(m^{2}-1)^{2}}\mathop{\cos\/}\nolimits mz%
\right)+\cdots.

For the corresponding expansions of \mathop{\mathrm{se}_{{m}}\/}\nolimits\!\left(z,q\right) for m=3,4,5,\dots change \mathop{\cos\/}\nolimits to \mathop{\sin\/}\nolimits everywhere in (28.6.26).

The radii of convergence of the series (28.6.21)–(28.6.26) are the same as the radii of the corresponding series for \mathop{a_{{n}}\/}\nolimits\!\left(q\right) and \mathop{b_{{n}}\/}\nolimits\!\left(q\right); compare Table 28.6.1 and (28.6.20).