Small transitive families of subspaces (Q1412948)

A family of subspaces of a complex separable Hilbert space is transitive if every bounded operator which leaves each of its members invariant is scalar. This article surveys some results concerning transitive families of small cardinality, and adds new ones. The minimum cardinality of a transitive family in finite dimensions (greater than 2) is 4. In infinite dimensions, a transitive pair of linear manifolds exists but the minimal cardinality of a transitive family of dense operator ranges or norm-closed subspaces is not known. However, a transitive family of dense operator ranges with 5 elements can be dounded, and so can a transitive family of norm-closed subspaces with 4 elements. In finite dimensions (\(> 1\)), three nest algebras (corresponding to maximal nests) can intersect in the scalar operators, but two cannot. It is not known if this is the case in infinite dimensions for maximal nests of type \(\omega + 1\). Four such nest algebras can intersect in the scalar operators.