A [linear operator,](linear%20operator) $A$ is [idempotent](Idempotence) if $A^2=A.$ Such operators are also called _projectors_ or _projection operators_ since they give rise to [projections.](Projection.md) # Idempotent matrices %%Check that all projectors are idempotent and whether all idempotent operators are projectors - and the answer is yes according to Halmo's finite dimensional vector spaces (pg 73 of Halmos).%% #MathematicalFoundations/Algebra/AbstractAlgebra/LinearAlgebra/Operators/Matrices