Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

I'm familiar with Householder reflections; they are a simple transformation that, given a normal vector, describes reflection in the hyperplane perpendicular to that vector.

But do Householder reflections describe the complete space of matrices describing reflection about the origin, or are there other reflections that cannot be described by Householder reflections?

share|improve this question

2 Answers 2

up vote 2 down vote accepted

Mariano's answer should be enough, if you read your link to Wikipedia carefully, but maybe I could add some hints.

If you want to do a reflection about a hyperplane containing the origin, you pick a unitary orthogonal vector to that hyperplane, $v$, and then write the vector $x$ you want to reflect about $[v]^\bot$ as a sum

$$ x = \lambda v + u $$

with $u \in [v]^\bot$. Now, you want to determine $\lambda \in \mathbb{R}$ in order this to be true:

$$ v \cdot x = \lambda v\cdot v + v\cdot u = \lambda \ . $$

So, necessarily

$$ x = (v\cdot x) v + u \ . $$

Hence, if you want to reflect $x$ about $[v]^\bot$, you just substract two times its $v$-component:

$$ P_v x = x - 2 (v\cdot x) v = x - 2 v (v^t x) = (I - 2vv^t ) x \ . $$

So any (orthogonal) reflexion about any hyperplane $[v]^\bot$ is of the form

$$ P_v = I - 2vv^t \ . $$

That is, a Housholder reflection.

share|improve this answer

Yes, they are all reflections.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.