How a Karnuagh Map Works in the Corners?

You can’t connect just diagonally anywhere.

Assuming you’re using a 4 term K map (ie http://upload.wikimedia.org/wikipedia/commons/0/02/K-map_6%2C8%2C9%2C10%2C11%2C12%2C13%2C14_anti-race.svg) You can connect all 4 corners (because you have a 2×2 square which you can define by 2 terms—not B, not D in that map) or connect two corners that lie in the same line (ie upper left and right, which you would be not B, not C, not D)

As I remember it, you can treat a K-Map like a torus. The sides connect and the top and bottom connect. So if you can reduce the four corners (as a single block) you are fine.

Another way to think of it is that you can “shift” the map at your convenience. I.e. you can shift it left, right, up and down. If you shift it to the left, the left most side jumps to the right. So you can do this to get things at the corners to be in the middle.

Someone correct me if I’m wrong.