Crap, someone just emailed me the answer: 5^([-1^n]*.5 + .5)
There is NO way in hell I would've been able to figure that out... I really hope we don't have these on the test. The time I've spent actually w/ pen and paper trying to figure this out has been well over 20 minutes, and spending 20 minutes on ONE problem is crazy.
[edit]
No, that won't work. I'd have to be {1, 5, 1, 5 ...}
...it would if it was n+1 though!
Damn this is frustrating.