“…These results have been subsequently extended into various directions. As a sample of the extensive literature on this topic we mention [15,44,45,46,56] (for the Witten Laplacian); see also [3,4,10,19,37,42,47,49,52,54] (for the Laplace-Beltrami operator), [17,31,51] (for the Hodge-de Rham Laplacian), and [11] (for sub-elliptic operators).…”