Definition:Idempotent Operator
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Definition
Let $\HH$ be a Hilbert space.
Let $A \in \map B \HH$ be a bounded linear operator.
Then $A$ is said to be (an) idempotent (operator) if and only if $A^2 = A$.
Also known as
Some sources refer to this concept as a projection.
However, another common convention (especially when dealing with Hilbert spaces) is to demand also that it be Hermitian.
In doing this, one arrives at what is called a projection on $\mathsf{Pr} \infty \mathsf{fWiki}$ in this context.
Also see
Sources
- 1990: John B. Conway: A Course in Functional Analysis (2nd ed.) ... (previous) ... (next) $\text {II}.3.1$