Fibonacci Number of Even Index by Golden Mean Modulo 1

Theorem
Let $n \in \Z$ be an integer.

Then:


 * $F_{2 n} \phi \bmod 1 = 1 - \phi^{-2 n}$
 * $F_n$ denotes the $n$th Fibonacci number
 * $\phi$ is the golden mean: $\phi = \dfrac {1 + \sqrt 5} 2$