A Lagrangian superposition principle for the continuity equation with reaction
Abstract
We study the continuity equation with reaction ∂tμ+ div(vμ) = wμ on [0,T]×ℝd, driven by Borel velocity and reaction fields v and w. We provide the first version of a Lagrangian superposition principle for the equation in its natural generality, assuming only finite quadratic energy without imposing boundedness conditions on the reaction term neither any form of compactness for the support of the measure. More precisely, every solution μ∈ 𝒞([0,T];ℳ+(ℝd)) with finite quadratic energy is represented by a probability measure η on absolutely continuous curves in the geometric cone over ℝd, concentrated on the solutions of the characteristic system x' = v(x), k' = w(x) k, through the $2$-homogeneous marginal h2t(η)=μt. We also establish the converse implication. The proof passes through a regularization of the triple (v,w,μ) which produces fields satisfying only local bounds; the core of the argument is therefore a representation theory under such local assumptions, whose key tool, of independent interest, is a two-time representation formula relating μs and μt along the flow of v for arbitrary times s,t.
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BibTeX
@misc{almi2026lagrangian,
title = {{A Lagrangian superposition principle for the continuity equation with reaction}},
author = {Stefano Almi and Giacomo Enrico Sodini},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.11300},
eprint = {2609.11300},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.11300},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Lagrangian superposition principle for the continuity equation with reaction
AU - Stefano Almi
AU - Giacomo Enrico Sodini
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.11300
UR - https://arxiv.org/abs/2609.11300
AB - We study the continuity equation with reaction ∂_tμ+ div(vμ) = wμ on [0,T]×ℝ^d, driven by Borel velocity and reaction fields v and w. We provide the first version of a Lagrangian superposition principle for the equation in its natural generality, assuming only finite quadratic energy without imposing boundedness conditions on the reaction term neither any form of compactness for the support of the measure. More precisely, every solution μ∈ 𝒞([0,T];ℳ_+(ℝ^d)) with finite quadratic energy is represented by a probability measure η on absolutely continuous curves in the geometric cone over ℝ^d, concentrated on the solutions of the characteristic system x' = v(x), k' = w(x) k, through the $2$-homogeneous marginal h^2_t(η)=μ_t. We also establish the converse implication. The proof passes through a regularization of the triple (v,w,μ) which produces fields satisfying only local bounds; the core of the argument is therefore a representation theory under such local assumptions, whose key tool, of independent interest, is a two-time representation formula relating μ_s and μ_t along the flow of v for arbitrary times s,t.
ER -