## general topology – The annihilator \$M^perp\$ of a set \$M neq emptyset\$ in an inner product space X is a closed subspace of X.

I’m trying to prove the following:

Show that the annihilator $$M^perp$$ of a set $$M neq emptyset$$ in an inner product space X is a closed subspace of X.

Next is the proof I have done, which indeed only proves that $$M^perp$$ can’t be open, from what I know, this doesn’t imply it is closed, so I need to ensure $$M^perp$$ is closed. I would really appreciate any corrections to this argument and if possible any orientation on how to prove the statement with basic concepts of the topic. Thanks.

Proof:

i) By definition
$$M^perp = {x in X: langle x,y rangle = 0 quad forall y in M}$$
Suppose $$x_1,x_2~in M^perp$$ and $$alpha$$ is a scalar. Then
$$langle alpha x_1+x_2, y rangle = alpha langle x_1,y rangle +langle x_2, yrangle = 0$$
This means $$~alpha x_1 +x_2 ~in M^perp$$, then $$~M^perp$$ is a subspace of X.

ii) Note that if $$M={vec{0}}$$, then $$M^perp = X$$ which is close and open, then in particular $$M^perp$$ is closed. Suppose then that M contains at least one element $$yneq vec{0}$$. On the other hand if $$M^perp = {vec{0}}$$, $$M^perp$$ is a singleton and therefore is closed. Suppose then that $$M^perp$$ contains at least one $$xneq vec{0}$$.

If $$M^perp$$ is an open subset of X and $$x_0 in M^perp$$ and $$x_0neq vec{0}$$, there exist a real number $$epsilon>0$$ such that the ball with the center in $$x_0$$ and radius $$epsilon$$ is contained in $$M^perp$$

$$B(x_0, epsilon)={xin X: ||x-x_0||
Now, consider the vector $$x’ = x_0+beta y~$$ where $$yin M$$, $$y neq vec{0}$$ and $$beta$$ is a scalar, it is true that we can choose $$beta$$ such that
$$||x’-x_0||= ||x_0+beta y-x_0||=||beta y||=|beta|~||y||
Since $$yin M rightarrow langle x_0, yrangle = 0$$, and therefore
$$langle x’,yrangle = langle x_0+beta y, yrangle = langle x_0, yrangle + betalangle y,y rangle = beta langle y,yrangle neq vec{0}. ~Since ~y neq vec{0}.$$
then $$x’ notin M^perp$$. In summary, we can always form $$x’$$ such that $$x’ in B(x_0,epsilon)$$ but $$x’ notin M^perp$$. This mean, $$B(x_0,epsilon) notsubset M^perp$$, which implies $$x_0$$ is not an interior point of $$M^perp$$, in consequence $$M^perp$$ is not open.

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