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A linear operator whose inverse is its adjoint is called unitary.
These operators can be thought of as generalizations of complex numbers
whose absolue value is 1.
A unitary operator preserves the ``lengths'' and ``angles'' between
vectors, and it can be considered as a type of rotation operator in
abstract vector space. Like Hermitian operators, the eigenvectors of
a unitary matrix are orthogonal. However, its eigenvalues are not