Power-kernel fractional Sturm--Liouville operators: a graph realization and boundary-independent singular-value asymptotics

Abstract

For $\tfrac12<\alpha<1$ we construct a closed realization of $\mathcal L u=\mathcal D_1^\alpha D_0^\alpha u$ in $L^2(0,1)$ whose domain contains the singular mode $x^{\alpha-1}$. The domain, defined via the distributional Caputo composition, is characterized by $I_0^{1-\alpha}u\in H^1(0,1)$ and $D_0^\alpha u-c\in\operatorname{ran}I_1^\alpha$; we prove a Volterra representation for its elements, graph-norm completeness, and that $\mathcal L$ is the adjoint of its zero-trace restriction. Four bounded endpoint traces give an ordinary boundary triplet, so all self-adjoint extensions correspond to Lagrangian planes in $\mathbb C^4$, including coupled endpoint conditions. For each we give an exact zero-eigenvalue criterion and, when $0\in\rho(\mathcal L_\omega)$, an inverse of the form $I_0^\alpha I_1^\alpha$ plus an explicit finite-rank correction. Sharp singular-value asymptotics $s_n(\mathcal L_\omega^{-1})=(\pi n)^{-2\alpha}\bigl(1+O(n^{-1})\bigr)$ yield $\mathcal L_\omega^{-1}\in\mathfrak S_p\iff p>1/(2\alpha)$, with weak-Schatten endpoint and Weyl asymptotics --- now for \emph{every} invertible boundary plane, not only the positive reference problem. As an application we obtain a mild-solution theorem for a Caputo-time diffusion equation in $C([0,T];L^2(0,1))$.