## Departmental PhD Thesis Exam – Qin Deng

Wednesday, August 25 2021
2:00 p.m. (sharp)

PhD Candidate: Qin Deng
Supervisor: Vitali Kapovitch
Thesis title:  Hölder continuity of tangent cones and non-branching in RCD(K,N) spaces

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This thesis is concerned with the study of the structure theory of metric measure spaces (X, d, m) satisfying the synthetic lower Ricci curvature bound condition RCD(K, N). We prove that such a space is non-branching and that tangent cones from the same sequence of rescalings are Holder continuous along the interior of every geodesic in X. More precisely, we show that the geometry of balls of small radius centred in the interior of any geodesic changes in at most a Holder continuous way along the geodesic in pointed Gromov-Hausdorff distance. This improves a result in the Ricci limit setting by Colding-Naber where the existence of at least one geodesic with such properties between any two points is shown. As in the Ricci limit case, this implies that the regular set of an RCD(K, N) space has m-a.e. constant dimension, a result recently established by Brue-Semola, and is m-a.e convex. It also implies that the top dimension regular set is weakly convex and, therefore, connected. In proving the main theorems, we develop in the RCD(K, N) setting the expected second
order interpolation formula for the distance function along the Regular Lagrangian flow of some vector field using its covariant derivative

A copy of the thesis can be found here:  Thesis Formatted