The residue is the coefficient of the $z^{-1}$ term in the Laurent Series. 

As you probably already know:

$$\cos z \equiv 1 - \frac{z^2}{2!} + \frac{z^4}{4!} - \frac{z^6}{6!} \pm \cdots $$

It follows that

$$\cos\left(z^{-1}\right) \equiv 1 - \frac{z^{-2}}{2!} + \frac{z^{-4}}{4!} - \frac{z^{-6}}{6!} \pm \cdots $$

Finally, multiplying through by $z$ gives:

$$z\cos\left(z^{-1}\right) \equiv z - \frac{z^{-1}}{2!} + \frac{z^{-3}}{4!} - \frac{z^{-5}}{6!} \pm \cdots $$

The coefficient of $z^{-1}$ is $-\frac{1}{2!} = -\frac{1}{2}$.

Finding residue of function
I need a validation rule that checks the date of a field is not within 4 working days of today's date. Is it possible to do this, what would the formula look like?
