Show that if and only if .
Anyways, I got from the LHS to the RHS but didn't find the restriction. Where should that pop up?
You must have made the same mistake I did the first part:
...and the mistake's already here! It must be
On the left hand:
On the right hand: ,and mulitiplying by and assuming the equality (as in the beginning) we continue:
putting and comparing real and imaginary parts I get as unique condition...