Set theory and cyclic vectors
  Let H be a separable, infinite dimensional Hilbert space and let S be a countable subset of H. Then most positive operators on H have the property that every nonzero vector in the span of S is cyclic, in the sense that the set of operators in the positive part of the unit ball of B(H) with this property is comeager for the strong operator topology.   Suppose \kappa is a regular cardinal such that \kappa \geq \omega_1 and 2^{<\kappa} = \kappa. Then it is relatively consistent with ZFC that 2^\omega = \kappa and for any subset S \subset H of cardinality less than \kappa the set of positive operators in the unit ball of B(H) for which every nonzero vector in the span of S is cyclic is comeager for the strong operator topology. 
Nik Weaver