Mixed Dimensional Compactness with Dimension Collapsing from Sn-1 Bundle Measures

Abstract

We provide a measure based topology for certain unions of C2 rectifiable submanifolds of mixed dimensions in Rn. In this topology lower dimensional sets remain in the limit as measures when higher dimensional sets collapse down to them. For example a decreasing sequence of spheres may have a limit consisting of just a point. The n-1 dimensional space of outward pointing vectors can be used for such a measure. It represents all C2 rectifiable sets of codimension at least one of Rn as rectifiable sets in RnXSn-1 with n-1 dimensional Hausdorff measure. When viewed as (n-1)-rectifiable varifolds or currents in RnXSn-1 they come equipped with compactness theorems. The projection of their limits to Rn recovers the rectifiable sets of mixed dimensions giving the limits for the desired topology on subsets of Rn. Both varifold and current compactness are required as there are sequences, such as honeycombs, that converge as varifolds but not as currents. Conversely sequences such as lifts of polyhedral approximations converge as currents but not as varifolds.