# Whittaker equation

A linear homogeneous ordinary differential equation of the second order:

$$w''+\left(\frac{1/4-\mu^2}{z^2}+\frac\lambda z-\frac14\right)w=0,\tag{*}$$

where the variables $z,w$ and the parameters $\lambda,\mu$ may take arbitrary complex values. Equation (*) represents the reduced form of a degenerate hypergeometric equation and was first studied by E.T. Whittaker [1]. For $\lambda=0$ the Whittaker equation is equivalent to the Bessel equation. If $2\mu$ is not an integer, a fundamental system of solutions of the Whittaker equation consists of the functions $M_{\lambda,\mu}(z)$ and $M_{\lambda,-\mu}(z)$; here $M_{\lambda,\mu}(z)$ is the Whittaker function (cf. Whittaker functions). For any value of the parameters the general solution of the Whittaker equation may be written in the form of a linear combination

$$w=C_1W_{\lambda,\mu}(z)+C_2W_{-\lambda,\mu}(-z),$$

where $W_{\lambda,\mu}(z)$ is the Whittaker function.

#### References

[1] | E.T. Whittaker, "An expression of certain known functions as generalized hypergeometric functions" Bull. Amer. Math. Soc. , 10 (1903) pp. 125–134 |

[2] | E.T. Whittaker, G.N. Watson, "A course of modern analysis" , Cambridge Univ. Press (1952) |

[3] | H. Bateman (ed.) A. Erdélyi (ed.) , Higher transcendental functions , 1. The gamma function. The hypergeometric functions. Legendre functions , McGraw-Hill (1953) |

[4] | A. Kratzer, W. Franz, "Transzendente Funktionen" , Akad. Verlagsgesell. Geest u. Portig K.-D. (1960) |

[5] | E. Kamke, "Differentialgleichungen: Lösungen und Lösungsmethoden" , 1. Gewöhnliche Differentialgleichungen , Akad. Verlagsgesell. (1942) |

**How to Cite This Entry:**

Whittaker equation.

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Whittaker_equation&oldid=32644