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28 Mathieu Functions and Hill’s EquationModified Mathieu Functions

§28.20 Definitions and Basic Properties

Contents

§28.20(i) Modified Mathieu’s Equation

When z is replaced by \pm iz, (28.2.1) becomes the modified Mathieu’s equation:

with its algebraic form

§28.20(ii) Solutions \mathop{\mathrm{Ce}_{{\nu}}\/}\nolimits, \mathop{\mathrm{Se}_{{\nu}}\/}\nolimits, \mathop{\mathrm{Me}_{{\nu}}\/}\nolimits, \mathop{\mathrm{Fe}_{{n}}\/}\nolimits, \mathop{\mathrm{Ge}_{{n}}\/}\nolimits

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

§28.20(iii) Solutions \mathop{{\mathrm{M}^{{(j)}}_{{\nu}}}\/}\nolimits

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

28.20.8h=\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 2ih\zeta}} as \zeta\to\infty in the respective sectors |\mathop{\mathrm{ph}\/}\nolimits\!\left(\mp 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 \mathop{{\mathrm{M}^{{(3)}}_{{\nu}}}\/}\nolimits\!\left(z,h\right), \mathop{{\mathrm{M}^{{(4)}}_{{\nu}}}\/}\nolimits\!\left(z,h\right) such that

as \realpart{z}\to+\infty with -\pi+\delta\leq\imagpart{z}\leq 2\pi-\delta, and

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

as \realpart{z}\to+\infty with |\imagpart{z}|\leq\pi-\delta.

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

28.20.13\mathop{{\mathrm{M}^{{(3)}}_{{\nu}}}\/}\nolimits\!\left(z,h\right)=\mathop{{%
\mathrm{M}^{{(1)}}_{{\nu}}}\/}\nolimits\!\left(z,h\right)+i\mathop{{\mathrm{M}%
^{{(2)}}_{{\nu}}}\/}\nolimits\!\left(z,h\right),

§28.20(iv) Radial Mathieu Functions \mathop{{\mathrm{Mc}^{{(j)}}_{{n}}}\/}\nolimits, \mathop{{\mathrm{Ms}^{{(j)}}_{{n}}}\/}\nolimits

For j=1,2,3,4,

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

§28.20(v) Solutions \mathop{\mathrm{Ie}_{{n}}\/}\nolimits, \mathop{\mathrm{Io}_{{n}}\/}\nolimits, \mathop{\mathrm{Ke}_{{n}}\/}\nolimits, \mathop{\mathrm{Ko}_{{n}}\/}\nolimits

28.20.17\mathop{\mathrm{Ie}_{{n}}\/}\nolimits\!\left(z,h\right)=i^{{-n}}\mathop{{%
\mathrm{Mc}^{{(1)}}_{{n}}}\/}\nolimits\!\left(z,ih\right),
28.20.18\mathop{\mathrm{Io}_{{n}}\/}\nolimits\!\left(z,h\right)=i^{{-n}}\mathop{{%
\mathrm{Ms}^{{(1)}}_{{n}}}\/}\nolimits\!\left(z,ih\right),
28.20.19
\mathop{\mathrm{Ke}_{{2m}}\/}\nolimits\!\left(z,h\right)=(-1)^{m}\tfrac{1}{2}%
\pi i\mathop{{\mathrm{Mc}^{{(3)}}_{{2m}}}\/}\nolimits\!\left(z,ih\right),
\mathop{\mathrm{Ke}_{{2m+1}}\/}\nolimits\!\left(z,h\right)=(-1)^{{m+1}}\tfrac{%
1}{2}\pi\mathop{{\mathrm{Mc}^{{(3)}}_{{2m+1}}}\/}\nolimits\!\left(z,ih\right),
28.20.20
\mathop{\mathrm{Ko}_{{2m}}\/}\nolimits\!\left(z,h\right)=(-1)^{m}\tfrac{1}{2}%
\pi i\mathop{{\mathrm{Ms}^{{(3)}}_{{2m}}}\/}\nolimits\!\left(z,ih\right),
\mathop{\mathrm{Ko}_{{2m+1}}\/}\nolimits\!\left(z,h\right)=(-1)^{{m+1}}\tfrac{%
1}{2}\pi\mathop{{\mathrm{Ms}^{{(3)}}_{{2m+1}}}\/}\nolimits\!\left(z,ih\right).

§28.20(vi) Wronskians

§28.20(vii) Shift of Variable

For n=0,1,2,\dots,

28.20.23
\mathop{{\mathrm{Mc}^{{(j)}}_{{2n}}}\/}\nolimits\!\left(z\pm\tfrac{1}{2}\pi i,%
h\right)=\mathop{{\mathrm{Mc}^{{(j)}}_{{2n}}}\/}\nolimits\!\left(z,\pm ih%
\right),
\mathop{{\mathrm{Ms}^{{(j)}}_{{2n+1}}}\/}\nolimits\!\left(z\pm\tfrac{1}{2}\pi i%
,h\right)=\mathop{{\mathrm{Mc}^{{(j)}}_{{2n+1}}}\/}\nolimits\!\left(z,\pm ih%
\right),
28.20.24
\mathop{{\mathrm{Mc}^{{(j)}}_{{2n+1}}}\/}\nolimits\!\left(z\pm\tfrac{1}{2}\pi i%
,h\right)=\mathop{{\mathrm{Ms}^{{(j)}}_{{2n+1}}}\/}\nolimits\!\left(z,\pm ih%
\right),
\mathop{{\mathrm{Ms}^{{(j)}}_{{2n+2}}}\/}\nolimits\!\left(z\pm\tfrac{1}{2}\pi i%
,h\right)=\mathop{{\mathrm{Ms}^{{(j)}}_{{2n+2}}}\/}\nolimits\!\left(z,\pm ih%
\right).

For s\in\Integer,

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).