Regularity Effects on the Small-Diffusion Asymptotics of the Principal Eigenvalue: Non-convergence and Convergence

We study the asymptotic behavior as \(d\to0^+\) of the principal eigenvalue \(λ(d)\) of the one-dimensional elliptic problem \[ \begin{cases} -dϕ''(x)-2αm'(x)ϕ'(x)+c(x)ϕ(x) =λ(d)ϕ(x), & x\in(a,b),\\ ϕ'(a)=ϕ'(b)=0, \end{cases} \] where \(α>0\) is fixed, \(m\in C^{1,1}([a,b])\), and \(c\in C([a,b])\). We assume that \(m'=0\) only at \(x_0\in(a,b)\), where \(m\) attains a strict minimum. Within this class, we construct a counterexample $m\in C^{1,1}$ for which \(λ(d)\) has two distinct subsequential limits as \(d\to0^+\), leading to the non-convergence of $λ(d)$. We then consider another class of $C^{1,1}$ advection functions for which $m'(x)/(x-x_0)$ has positive one-sided limits at $x_0$. For this class, we prove that $λ(d)$ converges as $d\to0^+$. Thus, \(C^{1,1}\) regularity alone does not ensure convergence of the principal eigenvalue, even when \(m\) has exactly one strict interior critical minimal point.