Complementary Projection is Projection
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Theorem
Let $\HH$ be a Hilbert space.
Let $A$ be a projection.
Then the complementary projection $I - A$ is also a projection.
Proof
By Characterization of Projections, $A$ is Hermitian.
Then $\paren {I - A}^* = I^* - A^* = I - A$ from Adjoining is Linear.
So $I - A$ is also Hermitian.
From Complementary Idempotent is Idempotent, $I - A$ is idempotent.
Hence, applying Characterization of Projections, $I - A$ is a projection.
$\blacksquare$