Intersections of translates of finite-dimensionally valued frame spaces are conditionally slice-full and almost slice-full

Abstract

In recent work, the topology of frame spaces ${\mathcal{F}}_{(X,\mu ),n}$ has been studied via Stiefel manifolds, revealing in particular a connectedness property for intersections of their translates when $\mathrm{span}(\{a_j{\}}_{j\in J}$ is not too large, in fact when $\mathrm{codim}(\mathrm{span}\{a_j^l{\}}_{(j,l)\in J\times [[1,n]]})\ge 3n$, where $\{a_j{\}}_{j\in J}$ is the translating family \cite{ElIdrissiKabbajMoalige2023}. The investigation of the connectedness of the intersections of translates of the frame space can be extended to questions about the algebro-geometric and measure-theoretic structure of such intersections. The present article addresses these questions by uncovering an almost-linear structure within intersections of translated frame spaces. We show that the set of non-frames in finite-dimensional Hilbert $C^{\ast }$-modules inherits the structure of a slice-wise real affine algebraic subvariety. As a consequence, it is a small subset in a precise measure-theoretic sense. In particular, we prove that for any finite-dimensional Hilbert $C^{\ast }$-module $\mathcal{H}$ and any countable collection of translates of the frame space ${\mathcal{F}}_{(X,\mu ),\mathcal{H}}$, the intersection is conditionally slice-full in $L^2(X,\mu ;\mathcal{H})$ and almost surely slice-full. We inform the reader that the notions of slice-wise real affine algebraic subvarieties (although related to ind-varieties), conditionally slice-full subsets and slice-full subsets (although related to shy sets) of a Hausdorff topological vector space are, to our knowledge, both new.

17 pages