Mathieu function

In mathematics, the Mathieu functions are solutions to the Mathieu differential equation, which is

${\displaystyle {\frac {d^{2}y}{dx^{2}}}+[a-2q\cos(2x)]y=0}$

The Mathieu functions are used in treating problems involving parametric resonance, vibrating elliptical drumheads, and gravitational waves in general relativity, among other applications. They were introduced by Emile Mathieu in 1868 in the context of the second problem.

Floquet solution

According to Floquet's theorem, for fixed values of a,q, Mathieu's equation admits a complex valued solution of form

${\displaystyle F(a,q,x)=\exp(i\mu )\,P(a,q,x)}$ 

where ${\displaystyle \mu }$  is a complex number, the Mathieu exponent, and P is a complex valued function which is periodic with period ${\displaystyle \pi }$ . However, P is in general not sinusoidal. In the example plotted below, ${\displaystyle a=1,\,q={\frac {1}{5}},\,\mu \approx 1+0.0995i}$  (real part, red; imaginary part; green):

 

Mathieu sine and cosine

For fixed a,q, the Mathieu cosine ${\displaystyle C(a,q,\xi )}$  is a function of ${\displaystyle \xi }$  defined as the unique solution of the Mathieu equation which

1. takes the value ${\displaystyle C(a,q,0)=1}$ ,
2. is an even function, or equivalently ${\displaystyle C^{\prime }(a,q,0)=0}$ .

Similarly, the Mathieu sine ${\displaystyle S(a,q,\xi )}$  is the unique solution which

1. takes the value ${\displaystyle S(a,q,0)=0}$ ,
2. is an odd function, or equivalently ${\displaystyle S^{\prime }(a,q,0)=0}$ .

These are closely related to the Floquet solution:

${\displaystyle C(a,q,x)={\frac {\exp(i\mu )}{F(a,q,0)}}\,{\frac {P(a,q,x)+P(a,q,-x)}{2}}}$ 

${\displaystyle S(a,q,x)={\frac {\exp(i\mu )}{F(a,q,0)}}\,{\frac {P(a,q,x)-P(a,q,-x)}{2}}}$ 

For small values of q, these functions do resemble sine and cosine near the origin. In the example plotted below (same values of a,q as above), the Mathieu cosine is plotted in red and cosine in green:

File:MathieuCosine example.gif

For this example, the McLaurin series is

${\displaystyle C(1,{\frac {1}{5}},x)=1-{\frac {3}{10}}\,x^{2}-{\frac {31}{600}}\,x^{4}+{\frac {1613}{9000}}\,x^{6}+O(x^{8})}$ 

Periodic solutions

For countably many special values of a (in terms of q), called eigenvalues, the Mathieu equation admits solutions which are periodic with period ${\displaystyle 2\pi }$ .

Symbolic computation engines

Various special functions related to the Mathieu functions are implemented in Maple and Mathematica.

External links

• Mathieu function at Mathworld (Wolfram Research).
• EqWorld offers a useful page on the Mathieu equation.

References

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