Transactions of the American Mathematical Society

Published by the American Mathematical Society since 1900, Transactions of the American Mathematical Society is devoted to longer research articles in all areas of pure and applied mathematics.

ISSN 1088-6850 (online) ISSN 0002-9947 (print)

The 2024 MCQ for Transactions of the American Mathematical Society is 1.48 .

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Transport of currents and geometric Rademacher-type theorems
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by Paolo Bonicatto, Giacomo Del Nin and Filip Rindler;
Trans. Amer. Math. Soc. 378 (2025), 4011-4075
DOI: https://doi.org/10.1090/tran/9312
Published electronically: March 19, 2025

Abstract:

The transport of many kinds of singular structures in a medium, such as vortex points/lines/sheets in fluids, dislocation loops in crystalline plastic solids, or topological singularities in magnetism, can be expressed in terms of the geometric (Lie) transport equation \[ \frac {\mathrm {d}}{\mathrm {d} t} T_t + \mathcal {L}_{b_t} T_t = 0 \] for a time-indexed family of integral or normal and usually boundaryless $k$-currents $t \mapsto T_t$ in an ambient space $\mathbb {R}^d$ (or a subset thereof). Here, $b_t = b(t,\cdot )$ is the driving vector field and $\mathcal {L}_{b_t} T_t$ is the Lie derivative of $T_t$ with respect to $b_t$ (introduced in this work via duality). Written in coordinates for different values of $k$, this PDE encompasses the classical transport equation ($k = d$), the continuity equation ($k = 0$), as well as the equations for the transport of dislocation lines in crystals ($k = 1$) and membranes in liquids ($k =d-1$). The top-dimensional and bottom-dimensional cases have received a great deal of attention in connection with the DiPerna–Lions and Ambrosio theories of Regular Lagrangian Flows. On the other hand, very little is rigorously known at present in the intermediate-dimensional cases. This work thus develops the theory of the geometric transport equation for arbitrary $k$ and in the case of boundaryless currents $T_t$, covering in particular existence and uniqueness of solutions, structure theorems, rectifiability, and a number of Rademacher-type differentiability results. The latter yield, given an absolutely continuous (in time) path $t \mapsto T_t$, the existence almost everywhere of a “geometric derivative”, namely a driving vector field $b_t$. This subtle question turns out to be intimately related to the critical set of the evolution, a new notion introduced in this work, which is closely related to Sard’s theorem and concerns singularities that are “smeared out in time”. Our differentiability results are sharp, which we demonstrate through an explicit example of a heavily singular evolution, which is nevertheless Lipschitz-regular in time.
References
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Bibliographic Information
  • Paolo Bonicatto
  • Affiliation: Dipartimento di Matematica, Università di Trento, Via Sommarive 14, 38123 Trento, Italy
  • MR Author ID: 1143565
  • ORCID: 0000-0001-9188-9113
  • Email: paolo.bonicatto@unitn.it
  • Giacomo Del Nin
  • Affiliation: Max Planck Institute for Mathematics in the Sciences, Inselstrasse 22, 04103 Leipzig, Germany
  • MR Author ID: 1440281
  • ORCID: 0000-0001-7308-9753
  • Email: giacomo.delnin@mis.mpg.de
  • Filip Rindler
  • Affiliation: Mathematics Institute, University of Warwick, Zeeman Building, CV4 7AL Coventry, United Kingdom
  • MR Author ID: 857689
  • ORCID: 0000-0003-2126-3865
  • Email: F.Rindler@warwick.ac.uk
  • Received by editor(s): March 16, 2023
  • Received by editor(s) in revised form: July 12, 2024
  • Published electronically: March 19, 2025
  • Additional Notes: This project had received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, grant agreement No 757254 (SINGULARITY)
  • © Copyright 2025 American Mathematical Society
  • Journal: Trans. Amer. Math. Soc. 378 (2025), 4011-4075
  • MSC (2020): Primary 49Q15, 35Q49
  • DOI: https://doi.org/10.1090/tran/9312