Advanced Linear Algebra Presentation

Stacey Lynn Baxter-Rienstra

email

sbaxter@cs.rice.edu ; rienstra@gauss.cl.uh.edu

Note: Due to the nature of HTML publishing the inner product notation will be square brackets instead of the pointed brackets.

Lemma

Let (V,|| ||) be a normed vector space.

Then || || comes from an inner product <=> the norm satisfies the parallelogram law

|| u + v ||^2 + || u-v||^2 = 2(||u||^2 + ||v||^2)

for all u,v in V.


=>If (V,|| ||) is a normed vector space by definition

V is a real vector space with a function

|| ||:V->R satisfying


RHS

If || || comes from an inner product then:

(||u+v||)^2 +(||u-v||)^2

[u+v,u+v] + [u-v,u-v] using the definition of a norm

[u,u]+[v,v]+[u,v]+[u,v]+[u,u]+[v,v]-[v,u]-[u,v] using bilinearity of [,] and symmetry.

[u,u]+[v,v]+[u,u]+[v,v]

2[u,u]+2[v,v]

2([u,u]+[v,v]) by using distributive property of R

2(||u||^2 + ||v||^2) by definition of a norm of a vector space


Therefore, the Lemma has been proven in the forward direction.