Regularized Subjective-Surface Flow with Monotone Reaction: Global Classical Well-Posedness and Stability

Abstract

Touching and dividing cell nuclei may appear as connected structures in microscopy images, making it difficult to distinguish neighboring nuclei during segmentation. We introduce and analyze a regularized subjective-surface model designed for this setting. On a smooth bounded domain $Ω\subset\mathbb{R}^n$ with homogeneous Dirichlet boundary conditions, the model is \[ u_t = νΔu + \left(\varepsilon^2+|\nabla u|^2\right)^{1/2} \operatorname{div}\!\left( G(x)\frac{\nabla u} {\left(\varepsilon^2+|\nabla u|^2\right)^{1/2}} \right) - μΛ(x)H_η(u-q). \] Here, $\varepsilon>0$ and $ν>0$ are fixed regularization parameters, $G$ is a strictly positive smooth edge coefficient, and the nonnegative interaction weight $Λ$ incorporates fixed information about neighboring nucleus candidates. The principal objective of this work is to establish an existence theory for the proposed model. For compatible $C^{2+α}$ initial data taking values in $[0,1]$, we prove the existence and uniqueness of a global classical solution whose restriction to every finite time interval is Schauder-classical, together with preservation of the physical range, finite-time Schauder estimates, and $L^\infty$-nonexpansive dependence on the initial data. The main analytical step is a global spatial-gradient bound, obtained by combining gradient estimates near $\partialΩ$ with interior gradient estimates. These results provide a mathematical foundation for applying the model to the analysis of touching and dividing nuclei in 3D and 3D+time microscopy image data.