# When does equality hold in Cauchy-Schwarz inequality

1. Sep 3, 2008

### Hitman2-2

1. The problem statement, all variables and given/known data
Prove that if V is a vector space over $$\mathbb{C}^n$$ with the standard inner product, then

$$|<x,y>| = ||x|| \cdot ||y||$$

implies one of the vectors x or y is a multiple of the other.

3. The attempt at a solution
Assume the identity holds and that y is not zero. Let

$$a = \frac {<x,y>} {||y||^2}$$

and let z = x - ay. I've shown that y and z are orthogonal and want to show

$$|a| = \frac {||x||} {||y||}$$

Well,

$$|a| \cdot ||y|| = \frac {|<x,y>|} {||y||} = \frac {|\sum_{i=1}^n a_i \overline{b_i} |} {\sqrt{\sum_{i=1}^n |b_i|^2}} = \sqrt{\frac {\left(\sum_{i=1}^n a_i \overline{b_i} \right) \left(\sum_{i=1}^n \overline{a_i} b_i \right)} {\sum_{i=1}^n |b_i|^2} }$$

but now I don't see how to simply this further to get this equal to the norm of x.

2. Sep 3, 2008

### Hitman2-2

Never mind ... I think I've got it. I've totally neglected that by assumption,

$$\frac { | \langle x,y \rangle | } { \| y \| } = \| x \|$$

so then it follows that

$$|a| \cdot \| y \| = \| x \|$$

Then since

$$\| x \|^2 = \| ay + z \|^2 = \| ay \|^2 + \| z \|^2 \Rightarrow \| z \| = 0$$

the result follows.

Embarassing.