1. Confirm that for f defined by (1.19a), the partial derivatives defined by (1.19b), (1.19c) and (1.19d)
are not continuous at the origin. Note that if these partial derivatives were continuous at the origin,
then from the above two results, f would be both differentiable and continuous at the origin, which
would be in contradiction to the result derived in § 1.2.2.