I have a question in a Hilbert Spaces course as follows: Let $X=(x_1, x_2)$ be vector in a vector space of all ordered pairs of complex numbers X. Can we obtain the norm defined on X by: $\|X\|=|x_1|+|x_2|$ from an inner product?
I know a norm comes from an inner product space if and only if it satisfies the parallelogram law given by $\|X+Y\|^2+\|X-Y\|^2=2\|X\|^2+2\|Y\|^2$. However, I am quite stuck using this method. Is there another way to solve it?