Unitary operator

In functional analysis, a unitary operator is a bounded linear operator U on a Hilbert space satisfying

U*U=UU*=I

where I is the identity operator. This property is equivalent to any of the following:

[itex]\langle Ux, Uy \rangle = \langle x, y \rangle.[itex]

Unitary matrices are precisely the unitary operators on finite-dimensional Hilbert spaces, so the notion of a unitary operator is a generalisation of the notion of a unitary matrix.

Unitary operators implement isomorphisms between operator algebras.

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