Riesz transforms for 1≤p≤2

It has been asked (see R. Strichartz, Analysis of the Laplacian..., J. Funct. Anal. 52 (1983), 48-79) whether one could extend to a reasonable class of non-compact Riemannian manifolds the L-p boundedness of the Riesz transforms that holds in R-n. Several partial answers have been given since. In the present paper, we give positive results for 1 less than or equal to p less than or equal to 2 under very weak assumptions, namely the doubling volume property and an optimal on-diagonal heat kernel estimate. In particular, we do not make any hypothesis on the space derivatives of the heat kernel. We also prove that the result cannot hold for p > 2 under the same assumptions. Finally, we prove a similar result for the Riesz transforms on arbitrary domains of R-n.