Estimates of Integral Kernels for Semigroups Associated with Second-Order Elliptic Operators with Singular Coefficients

In this paper we obtain pointwise two-sided estimates for the integral kernel of the semigroup associated with second-order elliptic differential operators –(a)+b ₁+b ₂+V with real measurable (singular) coefficients, on an open set R ^N . The assumptions we impose on the lower-order terms allow for the case when the semigroup exists on L ^p () for p only from an interval in 1,), neither enjoys a standard Gaussian estimate nor is ultracontractive in the scale L ^p (). We show however that the semigroup is ultracontractive in the scale of weighted spaces L ^p (,²dx) with a suitable weight and derive an upper and lower bound on its integral kernel.