# Orthogonal operator and reflection

1. Jun 26, 2014

### drawar

1. The problem statement, all variables and given/known data
Let $n$ be a unit vector in $V$ . Define a linear operator $F_n$ on $V$ such that
$$F_n(u) = u-2\langle u, n \rangle n \; \mathrm{for} \; u \in V.$$
$F_n$ is called the reflection on $V$ along the direction of $n$. Let $S$ be an orthogonal linear operator on $V$ and let $W = \left\{u \in V | S(u) = u\right\}$. Suppose $W^\perp$ is not empty and $w$ is a nonzero vector in $W^\perp$.

(a) Find a unit vector $n$ such that $F_n(S(w))=w$.

(b) Prove that $W \subsetneq E_1(F_n \circ S)$, where $n$ is the unit vector obtain in (a) and $E_1(F_n \circ S)$ denotes the eigenspace of $F_n \circ S$ associated with 1.

2. Relevant equations
$S$ is orthogonal iff $||S(u)||=||u||$ for all $u \in V$

3. The attempt at a solution
Okay I admit I have no idea how to do either of the questions but I would really appreciate some help to do (a) because without getting it done first, I wouldn't be able to proceed to (b). So right now all I can do for (a) is writing down the equation I'm supposed to solve, hoping something useful would pop up and lead me straight to the answer but it doesn't seem that easy:
$$S(w)-2 \langle S(w),n \rangle n = w.$$
Honestly I'm stuck at here, is there any way I can do about it?

2. Jun 28, 2014

### bloby

Perhaps not the best way but: what is $F_n^2$, how can (a) be restated? And is there a way to recover n knowing v and $F_n(v)$?

3. Jun 29, 2014

### drawar

$F_n^2=I$, so the equation in (a) reduces to $S(w)=F_n(w)$ (- I guess this requires $F_n$ to be bijective or at least injective but I don't know how to prove it). From here $n$ can be found by subtracting $F_n(w)$ (or equivalently $S(w)$) from $w$ followed by a normalization. Does that sound ok?

4. Jun 29, 2014

### bloby

Yes. Either verify the solution found or simply use the definition of injective.

Yes.

Is the part (b) ok?

Last edited: Jun 29, 2014
5. Jun 29, 2014

### drawar

No, unfortunately. Would appreciate further hints if possible!

6. Jun 30, 2014

### bloby

I gave perhaps too much hints for (a): it's not magical trick, I just figured out what $F_n$ does on a vector in $\mathbb R^3$, (do a sketch) hence the name and the intuitions.

For (b) there are two things to prove: $W \subset E_1(F_n \circ S)$ and there is a vector $\not\in W$ that belongs to $E_1(F_n \circ S)$.

What does it means for a vector v to be in $E_1(F_n \circ S)$?

7. Jul 1, 2014

### drawar

$E_1(F_n \circ S)=\left\{w \in V|F_n(S(w))=w\right\}$. $W \subset E_1(F_n \circ S)$ is because of the fact that $S$ is orthogonal. It remains to find a vector that $\not\in W$ but belongs to $E_1(F_n \circ S)$, which is nothing but $w$.

Last edited: Jul 1, 2014
8. Jul 1, 2014

### bloby

Yes, yes, and the definition of W and the fact that $w$ of (a) belongs to $W^\perp$

9. Jul 1, 2014

### drawar

Yeah, that also. Thanks for being so patient with me. I really appreciate your instructive help and guidance.

10. Jul 1, 2014

### bloby

You're welcome