Characterization of Unitary Operators

Theorem

Let $\HH$ be a Hilbert space.

Let $A$ be a bounded linear operator on $\HH$.

Then the following are equivalent:

$(1): \quad A$ is a unitary operator

$(2): \quad A^* A = A A^* = I$, where $A^*$ denotes the adjoint of $A$, and $I$ denotes the identity operator

$(3): \quad A$ is a normal isometry

Sources

• 1990: John B. Conway: A Course in Functional Analysis ... (previous) ... (next) $\text {II}.2.18$