Right, but I don't understand what you mean in post #6.
What is clear to me so far is this: For any n ∈ Z, n can be written in the following way, n = x*y + r, where x,y,r ∈ Z also.
Now given (in this example) that n != 4q+1 and n != 4q+3, (this is where I'm confused), Why wouldn't it follow...