Why not just
?
Actually, the above equation doesnt quite come out correctly, I beleive it has the gaps in the proper places, but I dont think it returns parabolic looking graph otherwise. Its allot to type into a calculator though, so I could've made syntax errors. I think its a step in the right direction though.
EDIT:
I regraphed the above equation, and it looks just like the parabola y=x^2 except for gaps on the intervals
and
I stand by my first equation I posted