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28 Mathieu Functions and Hill’s EquationModified Mathieu Functions

§28.20 Definitions and Basic Properties

Contents
  1. §28.20(i) Modified Mathieu’s Equation
  2. §28.20(ii) Solutions \operatorname{Ce}_{\nu}, \operatorname{Se}_{\nu}, \operatorname{Me}_{\nu}, \operatorname{Fe}_{n}, \operatorname{Ge}_{n}
  3. §28.20(iii) Solutions {\operatorname{M}^{(j)}_{\nu}}
  4. §28.20(iv) Radial Mathieu Functions {\operatorname{Mc}^{(j)}_{n}}, {\operatorname{Ms}^{(j)}_{n}}
  5. §28.20(v) Solutions \operatorname{Ie}_{n}, \operatorname{Io}_{n}, \operatorname{Ke}_{n}, \operatorname{Ko}_{n}
  6. §28.20(vi) Wronskians
  7. §28.20(vii) Shift of Variable

§28.20(i) Modified Mathieu’s Equation

When z is replaced by \pm\mathrm{i}z, (28.2.1) becomes the modified Mathieu’s equation:

28.20.1 w^{\prime\prime}-\left(a-2q\cosh\left(2z\right)\right)w=0,

with its algebraic form

§28.20(ii) Solutions \operatorname{Ce}_{\nu}, \operatorname{Se}_{\nu}, \operatorname{Me}_{\nu}, \operatorname{Fe}_{n}, \operatorname{Ge}_{n}

28.20.3 \operatorname{Ce}_{\nu}\left(z,q\right)=\operatorname{ce}_{\nu}\left(\pm%
\mathrm{i}z,q\right),\nu\neq-1,-2,\dots,
28.20.4 \operatorname{Se}_{\nu}\left(z,q\right)=\mp\mathrm{i}\operatorname{se}_{\nu}%
\left(\pm\mathrm{i}z,q\right),\nu\neq 0,-1,\dots,
28.20.5 \operatorname{Me}_{\nu}\left(z,q\right)=\operatorname{me}_{\nu}\left(-\mathrm{%
i}z,q\right),
28.20.6 \operatorname{Fe}_{n}\left(z,q\right)=\mp\mathrm{i}\operatorname{fe}_{n}\left(%
\pm\mathrm{i}z,q\right),n=0,1,\dots,
28.20.7 \operatorname{Ge}_{n}\left(z,q\right)=\operatorname{ge}_{n}\left(\pm\mathrm{i}%
z,q\right),n=1,2,\dots.

§28.20(iii) Solutions {\operatorname{M}^{(j)}_{\nu}}

Assume first that \nu is real, q is positive, and a=\lambda_{\nu}\left(q\right); see §28.12(i). Write

28.20.8 h=\sqrt{q}\;(>0).

Then from §2.7(ii) it is seen that equation (28.20.2) has independent and unique solutions that are asymptotic to \zeta^{\ifrac{1}{2}}e^{\pm 2\mathrm{i}h\zeta} as \zeta\to\infty in the respective sectors |\operatorname{ph}\left(\mp\mathrm{i}\zeta\right)|\leq\tfrac{3}{2}\pi-\delta, \delta being an arbitrary small positive constant. It follows that (28.20.1) has independent and unique solutions {\operatorname{M}^{(3)}_{\nu}}\left(z,h\right), {\operatorname{M}^{(4)}_{\nu}}\left(z,h\right) such that

as \Re z\to+\infty with -\pi+\delta\leq\Im z\leq 2\pi-\delta, and

as \Re z\to+\infty with -2\pi+\delta\leq\Im z\leq\pi-\delta. See §10.2(ii) for the notation. In addition, there are unique solutions {\operatorname{M}^{(1)}_{\nu}}\left(z,h\right), {\operatorname{M}^{(2)}_{\nu}}\left(z,h\right) that are real when z is real and have the properties

as \Re z\to+\infty with |\Im z|\leq\pi-\delta.

For other values of z, h, and \nu the functions {\operatorname{M}^{(j)}_{\nu}}\left(z,h\right), j=1,2,3,4, are determined by analytic continuation. Furthermore,

28.20.13 {\operatorname{M}^{(3)}_{\nu}}\left(z,h\right)={\operatorname{M}^{(1)}_{\nu}}%
\left(z,h\right)+\mathrm{i}{\operatorname{M}^{(2)}_{\nu}}\left(z,h\right),
28.20.14 {\operatorname{M}^{(4)}_{\nu}}\left(z,h\right)={\operatorname{M}^{(1)}_{\nu}}%
\left(z,h\right)-\mathrm{i}{\operatorname{M}^{(2)}_{\nu}}\left(z,h\right).

§28.20(iv) Radial Mathieu Functions {\operatorname{Mc}^{(j)}_{n}}, {\operatorname{Ms}^{(j)}_{n}}

For j=1,2,3,4,

28.20.15 {\operatorname{Mc}^{(j)}_{n}}\left(z,h\right)={\operatorname{M}^{(j)}_{n}}%
\left(z,h\right),n=0,1,\dots,
28.20.16 {\operatorname{Ms}^{(j)}_{n}}\left(z,h\right)=(-1)^{n}{\operatorname{M}^{(j)}_%
{-n}}\left(z,h\right),n=1,2,\dots.

§28.20(v) Solutions \operatorname{Ie}_{n}, \operatorname{Io}_{n}, \operatorname{Ke}_{n}, \operatorname{Ko}_{n}

28.20.17 \operatorname{Ie}_{n}\left(z,h\right)={\mathrm{i}}^{-n}{\operatorname{Mc}^{(1)%
}_{n}}\left(z,\mathrm{i}h\right),
28.20.18 \operatorname{Io}_{n}\left(z,h\right)={\mathrm{i}}^{-n}{\operatorname{Ms}^{(1)%
}_{n}}\left(z,\mathrm{i}h\right),
28.20.19
\operatorname{Ke}_{2m}\left(z,h\right)=(-1)^{m}\tfrac{1}{2}\pi\mathrm{i}{%
\operatorname{Mc}^{(3)}_{2m}}\left(z,\mathrm{i}h\right),
\operatorname{Ke}_{2m+1}\left(z,h\right)=(-1)^{m+1}\tfrac{1}{2}\pi{%
\operatorname{Mc}^{(3)}_{2m+1}}\left(z,\mathrm{i}h\right),

§28.20(vi) Wronskians

§28.20(vii) Shift of Variable

For s\in\mathbb{Z},

When \nu is an integer the right-hand sides of (28.20.25) are replaced by the their limiting values. And for the corresponding identities for the radial functions use (28.20.15) and (28.20.16).