Answer
$a_{n}$ = $sin(n\pi/2)$
Work Step by Step
We can see that every odd term is either 1 or -1 and all even terms are 0. This hints that we are dealing with a trigonometric function with relation to $\pi$. The oscillation of this sequence matches a $sin$ graph, as $sin(pi/2)$ = 1, $sin(2pi/2) = 0, sin(3pi/2) = -1 $ and so forth.
