Complementary Projection of Complementary Projection is Projection

Theorem
Let $\HH$ be a Hilbert space.

Let $I$ be an identity operator on $\HH$.

Let $A$ be a projection.

Let $B$ be the complementary projection of A.

Then the complementary projection of $B$ is $A$.

Proof
From Complementary Projection is Projection the complementary projection of $B$ is well-defined.

Let $C$ be the complementary projection of $B$.

We have: