The Kato square root property for fractional elliptic operators in divergence form

Abstract

We establish the Kato square root property for fractional elliptic operators $L = -\operatorname{div}^α(μ\nabla^α\cdot)$ of order $2α\in (0,2)$ on $\mathbb{R}^d$, built from the Riesz fractional gradient and divergence, with bounded, measurable and uniformly elliptic complex coefficients $μ$. More precisely, we show that the domain of $L^{1/2}$ coincides with the distributional fractional Sobolev space $\mathrm{S}^{α,2}(\mathbb{R}^d)$ and that $\|L^{1/2} u\|_{\mathrm{L}^2} \simeq \|\nabla^αu\|_{\mathrm{L}^2}$, with constants that are locally uniform in $α$. As a consequence, we recover the Kato square root property for elliptic operators $-\operatorname{div}(μ\nabla\cdot)$ in the limit $α\to 1$, and we obtain continuity of the square root with respect to the parameter $α$. The proof follows the second-order approach to the Kato problem. Its two new ingredients are nonlocal off-diagonal estimates of polynomial order for the resolvents of $L$, and a construction of $T(b)$-test functions that circumvents the nonlocality of $\nabla^α$ by factorizing it through Riesz potentials.