I am having trouble understanding the following excerpt from a math text I'm working through:
My question specifically is how line 2 came about in the expansion. How do the real and imaginary parts expand like that? Also I can't figure out from the context what the cursive "I" means.
For reference, I understand how to expand the 2-norm squared of $||x + iy||^2 = x^2 + y^2$, but I can't seem to understand how it's working in this example.