If z is a complex variable $\displaystyle z\in \mathbb{C}$

Prove that:

$\displaystyle e^{\frac{1}{2}a(z-1/z)}=\sum _{n=-\infty }^{\infty } J_n(a)z^n$

where:

$\displaystyle J_n(a)=\frac{1} {2\pi } \int _0^{2\pi }\text {cos} ({n*$\theta $} - a *{sin}(\theta )) d\theta$