Whilst writing out revision notes, something struck me!
let be a finite dimensional real inner product space.
let be a subspace of V.
Definition
The annihilator of , denoted where is the dual space of V.
Definition
The perpendicular space of , denoted
It might already be apparent that there is some connection between these two – both involve things going to zero, whenever something is done to everything in .
To make this more explicit, first we need a theorem.
Theorem: (weaker version of) Riesz Representation Theorem
is an isomorphism of vector spaces. Restated, we can associate each linear map uniquely with a vector such that .
Proof:
Linearity:
let
and hence we see this is a linear map.
Injectivity:
Suppose
then
so specifically
thus by positive definiteness, hence is injective.
Surjectivity:
Since we have that this is an injective linear map between spaces of the same dimension, this follows by Rank Nullity. Alternatively:
let
take a basis
then define
and we can check that this works.
Now to use this theorem to show the connection.
but for some .
thus
So we have the result we wanted:
Applications
This means all of those oh-so-fun identities one derives for annihilator subspace structure can be passed over to perpendicular subspace structure!
Example:
Given the identity
we can deduce
linearity of gives us
so by injectivity of we have
i.e. compare
to