Definition (Cyclic, Separating, Standard Subspace) A closed real linear subspace H of a complex Hilbert space H is called cyclic, if H+iH is dense in H, separating, if H∩iH={0}, and standard, if H is cyclic and separating.
Definition (Cyclic, Separating, Standard Subspace)
A closed real linear subspace H of a complex Hilbert space H is called