# Jacobi–Anger expansion

In mathematics, the Jacobi–Anger expansion (or Jacobi–Anger identity) is an expansion of exponentials of trigonometric functions in the basis of their harmonics. It is useful in physics (for example, to convert between plane waves and cylindrical waves), and in signal processing (to describe FM signals). This identity is named after the 19th-century mathematicians Carl Jacobi and Carl Theodor Anger.

The most general identity is given by:[1][2]

$e^{i z \cos \theta} = \sum_{n=-\infty}^{\infty} i^n\, J_n(z)\, e^{i n \theta},$

where $J_n(z)$ is the $n$-th Bessel function of the first kind and $i$ is the imaginary unit, $i^2=-1.$ Consequently:

$e^{i z \sin \theta} = \sum_{n=-\infty}^{\infty} J_n(z)\, e^{i n \theta}.$

Using the relation $J_{-n}(z) = (-1)^n\, J_{n}(z),$ valid for integer $n$, the expansion becomes:[1][2]

$e^{i z \cos \theta}=J_0(z)\, +\, 2\, \sum_{n=1}^{\infty}\, i^n\, J_n(z)\, \cos\, (n \theta).$

## Real-valued expressions

The following real-valued variations are often useful as well:[3]

\begin{align} \cos(z \cos \theta) &= J_0(z)+2 \sum_{n=1}^{\infty}(-1)^n J_{2n}(z) \cos(2n \theta), \\ \sin(z \cos \theta) &= -2 \sum_{n=1}^{\infty}(-1)^n J_{2n-1}(z) \cos\left[\left(2n-1\right) \theta\right], \\ \cos(z \sin \theta) &= J_0(z)+2 \sum_{n=1}^{\infty} J_{2n}(z) \cos(2n \theta), \\ \sin(z \sin \theta) &= 2 \sum_{n=1}^{\infty} J_{2n-1}(z) \sin\left[\left(2n-1\right) \theta\right]. \end{align}

## Notes

1. ^ a b Colton & Kress (1998) p. 32.
2. ^ a b Cuyt et al. (2008) p. 344.
3. ^ Abramowitz & Stegun (1965) p. 361, 9.1.42–45