Cosine to Power of Even Integer

Also defined as
This result is also reported in a less elegant form as:
 * $\ds \cos^n \theta = \frac 1 {2^{n - 1}} \sum_{k \mathop = 0}^{n / 2} \paren {\binom n k \map \cos {n - 2 k} \theta}$

for all even $n$.