1. The problem statement, all variables and given/known data 2. Relevant equations My main problem is connected with b/ and d/. I found a formula involving the norm of the function, but I'm not sure if it's a good idea using it. 3. The attempt at a solution I can prove that a function is not continuous by finding different values for x and y for which the limit at the point is approaching different values, but have no idea how to prove that it is continuous.