Doesn't always take a Mathematician to show it. Until (within a few years) recently, there was a district (15, I think - but unsure) in Illinois that looked like a horse shoe. There was a district that was drawn east/west that surrounded another entirely in it's center, and it was only connected by an uninhabited area down the center of a tollway. That was the only thing that connected the north and south areas of the district. It was obviously drawn to ensure a Latino Representative was in the Congress. I don't know how you'd prove it, but it was obvious looking at the map of the district & it's demographics. I can't find a link to prove it, because, of course, the district lines have been redrawn.