Kato’s square root problem in Banach spaces

Let L be an elliptic differential operator with bounded measurable coefficients, acting in Bochner spaces Lp(Rn;X) of X -valued functions on Rn.We characterize Kato’s square root estimates √Lu p ∇u p and the H∞-functional calculus of L in terms of R-boundedness properties of the resolvent of L, when X is a Banach function lattice with the UMD property, or a noncommutative Lp space. To do so, we develop various vector-valued analogues of classical objects in Harmonic Analysis, including a maximal function for Bochner spaces. In the special case X = C, we get a new approach to the Lp theory of square roots of elliptic operators, as well as an Lp version of Carleson’s inequality. © 2007 Elsevier Inc. All rights reserved.