抄録
Normal operators N satisfying \[{\mathfrak{A}_N} = {\mathfrak{A}''_N}\] are characterized in terms of invariant subspaces. It is shown that non-unitary isometries V always satisfy \[{\mathfrak{A}_V} = {\mathfrak{A}''_V}\]. Thus, since a unitary operator is normal, a complete description of isometries satisfying a double commutant theorem is achieved.
