The eigentheory for nonlocal cooperative-advective system and its role in the study of free boundary system for directional epidemic models

Abstract

In this paper, we propose and analyze a nonlocal cooperative reaction--diffusion system with free boundaries and drift terms, motivated by directional epidemic spread. Lacking a variational structure but requiring sharper regularity of solutions, the model poses substantial analytical challenges compared with previous works~\cite{Du,Berestycki2016a,Berestycki2016b,Cao2019,NguyenVo2022,Tang2024a,Tang2024b}. We first establish the well-posedness of the local problem and the global existence and uniqueness of classical solutions in $C^1$ space. We then study the associated nonlocal eigenvalue problem, proving the existence, simplicity, qualitative properties, and asymptotic behavior of the principal eigenvalue. The analysis employs Fredholm theory, the Crandall--Rabinowitz bifurcation theorem, and Hadamard-type derivative formulas to describe its parameter dependence and connection with the basic reproduction number~$R_0$. Building on this spectral characterization, we show that the system admits a \emph{sharp vanishing--spreading dichotomy} in its long-term dynamics. When $R_0\le 1$, all solutions vanish; for $R_0>1$, the outcome depends on the initial domain size~$h_0$ and the free-boundary expansion rate~$\mu$. There exists a critical habitat length~${\mathcal{L}}^{\ast }$ such that if $h_0<{\mathcal{L}}^{\ast }$, a threshold $\hat{\mu }>0$ separates vanishing ($\mu \in (0,\hat{\mu }]$) from spreading ($\mu >\hat{\mu }$). In the spreading regime, solutions converge to the unique positive steady state, while in the vanishing regime they decay uniformly to zero. These results provide a rigorous framework for the threshold dynamics of cooperative--advective nonlocal systems and offer mathematical insight for further studies in epidemic modeling, ecological invasion, and population dynamics.