Intersection of Orthocomplements is Orthocomplement of Closed Linear Span

Theorem
Let $H$ be a Hilbert space.

Let $\family {M_i}_{i \mathop \in I}$ be an $I$-indexed family of closed linear subspaces of $H$.

Then:


 * $\ds \bigcap_{i \mathop \in I} M_i^\perp = \paren {\vee \set {M_i : i \in I} }^\perp$

where:
 * $\perp$ denotes orthocomplementation
 * $\vee$ denotes closed linear span.

Corollary
Furthermore, the following equality holds:


 * $\ds \paren {\bigcap_{i \mathop \in I} M_i}^\perp = \vee \set {M_i^\perp : i \in I}$

Proof
By definition of set equality, it suffices to prove the following two inclusions:


 * $\ds \bigcap_{i \mathop \in I} M_i^\perp \subseteq \paren {\vee \set {M_i : i \in I} }^\perp$
 * $\paren {\vee \set {M_i : i \in I} }^\perp \subseteq \ds \bigcap_{i \mathop \in I} M_i^\perp$

$\ds \bigcap_{i \mathop \in I} M_i^\perp$ is contained in $\paren {\vee \set {M_i : i \in I} }^\perp$
By Orthocomplement is Closed Linear Subspace and Closed Linear Subspaces Closed under Intersection, both spaces considered are closed linear subspaces of $H$.

By Orthocomplement Reverses Subset, the required containment is equivalent to:


 * $\vee \set {M_i : i \in I} \subseteq \ds \paren {\bigcap_{i \mathop \in I} M_i^\perp }^\perp$

For $h \in \ds \bigcap_{i \mathop \in I} M_i^\perp$, by definition one has:
 * $h \perp M_i$ for all $i \in I$

That is:
 * $M_i \perp \ds \bigcap_{i \mathop \in I} M_i^\perp$

This is equivalent to saying that:
 * $M_i \subseteq \ds \paren {\bigcap_{i \mathop \in I} M_i^\perp }^\perp$

Definition $(2)$ of closed linear span now grants the desired subset relation.

$\paren {\vee \set {M_i : i \in I} }^\perp$ is contained in $\ds \bigcap_{i \mathop \in I} M_i^\perp$
By definition $(2)$ of closed linear span:
 * $\forall i \in I: M_i \subseteq \vee \set {M_i^\perp : i \in I}$

By Orthocomplement Reverses Subset, it follows that:


 * $\forall i \ni I: \paren {\vee \set {M_i : i \in I} }^\perp \subseteq M_i^\perp$

Therefore, by definition of set intersection:


 * $\paren {\vee \set {M_i : i \in I} }^\perp \subseteq \displaystyle \bigcap_{i \mathop \in I} M_i^\perp$

Thus we have established that:
 * $h \in \paren {\vee \set {M_i : i \in I} }^\perp \iff h \in \ds \bigcap_{i \mathop \in I} M_i^\perp$

From the definition of set equality, it follows that:
 * $\ds \bigcap_{i \mathop \in I} M_i^\perp = \paren {\vee \set {M_i : i \in I} }^\perp$

Proof of Corollary
From Orthocomplement is Closed Linear Subspace, the $M_i^\perp$ are an $I$-indexed family of closed linear subspaces of $H$.

The main result gives:


 * $\ds \bigcap_{i \mathop \in I} \paren {M_i^\perp}^\perp = \paren {\vee \set {M_i^\perp : i \in I} }^\perp$

Taking the orthocomplement of both sides, and using Corollary to Double Orthocomplement is Closed Linear Span yields the result.