c © Copyright by Theta, 2004 SET THEORY AND CYCLIC VECTORS

Abstract. 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 κ is a regular cardinal such that κ> ω1 and 2<κ = κ. Then it is relatively consistent with ZFC that 2ω = κ and for any subset S ⊂ H of cardinality less than κ 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