A Lagrangian superposition principle for the continuity equation with reaction
We study the continuity equation with reaction $\partial_t\mu + \operatorname{div}(v\mu) = w\mu$ on $[0,T]\times\mathbb{R}^d$, 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 $\mu \in \mathcal{C}([0,T];\mathscr{M}_+(\mathbb{R}^d))$ with finite quadratic energy is represented by a probability measure $\eta$ on absolutely continuous curves in the geometric cone over $\mathbb{R}^d$, concentrated on the solutions of the characteristic system $x' = v(x)$, $k' = w(x)\,k$, through the $2$-homogeneous marginal $h^2_t(\eta)=\mu_t$. We also establish the converse implication. The proof passes through a regularization of the triple $(v,w,\mu)$ 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 $\mu_s$ and $\mu_t$ along the flow of $v$ for arbitrary times $s,t$.