On the Weak Contractibility of the Space of Pure States

Abstract

We prove that the space $\mathscr{P}(\mathfrak{A})$ of pure states of a nonelementary, simple, separable, real rank zero $C^*$-algebra $\mathfrak{A}$ has trivial homotopy groups of all orders when $\mathscr{P}(\mathfrak{A})$ is equipped with the weak* topology. The convex-valued and finite-dimensional selection theorems of Michael are used to deform a family of pure states via the action of a homotopy of unitaries so that the entire family evaluates to one on a given projection $P \in \mathfrak{A}$. Then, the excision theorem of Akemann, Anderson, and Pedersen is used to iterate this deformation for a sequence of projections in $\mathfrak{A}$ excising a base point of the family of pure states, thereby contracting the family to the base point. Finally, we compare our weak contractibility result to the spaces of pure states of commutative $C^*$-algebras and rational rotation algebras, and compute the homotopy groups of the latter in terms of the homotopy groups of spheres.