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The next one is from
[1]
. To answer it you need to know that a matrix \(A\) is
involutory if \(A^2=I\text{,}\)
nilpotent if \(A^k=0\) for some integer \(k>0\)
a
projector
(also known as
idempotent
) if \(A^2=A\)
in-context